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Flutter - NoSuchMethodError: The method '[]' was called on null

 原文  2019-12-16 20:12:57  1   1    firebase/ flutter/ dart/ google-cloud-firestore/ widget

Question

I am encountering an error while trying to fetch data from Firestore using FutureBuilder. If a user logs in and I print (snapshot.data), I get a correct snapshot been pulled from the firestore:

flutter: FirebaseUser({displayName: null, providerId: Firebase, uid: UFubdB2xPCTUu8ESgQikOr9WOe52, phoneNumber: null, isEmailVerified: false, providerData: [{email: titi@momo.com, providerId: password, photoUrl: null, displayName: null, uid: titi@momo.com, phoneNumber: null}], email: titi@momo.com, photoUrl: null, lastSignInTimestamp: 1576367769805, isAnonymous: false, creationTimestamp: 1576367424402})

However if I navigate to the Feed screen, it throws an exception when i print (snapshot.data['email'])

flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following NoSuchMethodError was thrown building FutureBuilder<DocumentSnapshot>(dirty, state:
flutter: _FutureBuilderState<DocumentSnapshot>#5d856):
flutter: The method '[]' was called on null.
flutter: Receiver: null
flutter: Tried calling: []("email")
flutter:
flutter: The relevant error-causing widget was:
flutter:   FutureBuilder<DocumentSnapshot> file:///Users/momo/Desktop/combine/lib/screens/feed.dart:23:13
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
flutter: #1      _FeedState.build.<anonymous closure> (package:combine/screens/feed.dart:26:30)
flutter: #2      _FutureBuilderState.build (package:flutter/src/widgets/async.dart)
flutter: #3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4334:27)
flutter: #4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4223:15)
flutter: #5      Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #6      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #7      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #8      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #9      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #10     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #11     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #12     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #13     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #14     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #15     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #16     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #17     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #18     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #19     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #20     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #21     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #22     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #23     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #24     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #25     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #26     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #27     ParentDataElement.mount (package:flutter/src/widgets/framework.dart:4617:11)
flutter: #28     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #29     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5551:32)
flutter: #30     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #31     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #32     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #33     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #34     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #35     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #36     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #37     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #38     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #39     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #40     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #41     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #42     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #43     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #44     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #45     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #46     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #47     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #48     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #49     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #50     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #51     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #52     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
flutter: #53     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #54     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #55     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #56     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #57     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #58     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #59     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #60     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #61     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
flutter: #62     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #63     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #64     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #65     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #66     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #67     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #68     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #69     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #70     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #71     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #72     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #73     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #74     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #75     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #76     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #77     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #78     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #79     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #80     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #81     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #82     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #83     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #84     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #85     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #86     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #87     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #88     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #89     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #90     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #91     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #92     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #93     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #94     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #95     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #96     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #97     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #98     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #99     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #100    StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #101    ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #102    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #103    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #104    SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
flutter: #105    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #106    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #107    SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
flutter: #108    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #109    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #110    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #111    Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #112    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #113    ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #114    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #115    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #116    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #117    Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #118    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #119    ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #120    ParentDataElement.mount (package:flutter/src/widgets/framework.dart:4617:11)
flutter: #121    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #122    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #123    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #124    Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #125    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #126    StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
flutter: #127    ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #128    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #129    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #130    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
flutter: #131    Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #132    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
flutter: #133    ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
flutter: #134    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
flutter: #135    Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
flutter: #136    SliverMultiBoxAdaptorElement.updateChild (package:flutter/src/widgets/sliver.dart:1288:36)
flutter: #137    SliverMultiBoxAdaptorElement.createChild.<anonymous closure> (package:flutter/src/widgets/sliver.dart:1273:20)
flutter: #138    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2412:19)
flutter: #139    SliverMultiBoxAdaptorElement.createChild (package:flutter/src/widgets/sliver.dart:1266:11)
flutter: #140    RenderSliverMultiBoxAdaptor._createOrObtainChild.<anonymous closure> (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:354:23)
flutter: #141    RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1823:58)
flutter: #142    PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:875:15)
flutter: #143    RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1823:13)
flutter: #144    RenderSliverMultiBoxAdaptor._createOrObtainChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:343:5)
flutter: #145    RenderSliverMultiBoxAdaptor.insertAndLayoutChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:489:5)
flutter: #146    RenderSliverFixedExtentBoxAdaptor.performLayout (package:flutter/src/rendering/sliver_fixed_extent_list.dart:255:17)
flutter: #147    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
flutter: #148    RenderSliverEdgeInsetsPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:134:11)
flutter: #149    _RenderSliverFractionalPadding.performLayout (package:flutter/src/widgets/sliver.dart:1165:11)
flutter: #150    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
flutter: #151    RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:410:13)
flutter: #152    RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1367:12)
flutter: #153    RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1285:20)
flutter: #154    RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1584:7)
flutter: #155    PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:844:18)
flutter: #156    RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:344:19)
flutter: #157    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:774:13)
flutter: #158    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:283:5)
flutter: #159    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1102:15)
flutter: #160    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1041:9)
flutter: #161    SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:957:5)
flutter: #165    _invoke (dart:ui/hooks.dart:259:10)
flutter: #166    _drawFrame (dart:ui/hooks.dart:217:3)
flutter: (elided 3 frames from package dart:async)
flutter:
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════
flutter: Another exception was thrown: NoSuchMethodError: The method '[]' was called on null.
flutter: titi@momo.com

However, if i print only the (snapshot.data) while on the Feed screen, it throws up a null before getting the instance.

flutter: null
flutter: null
flutter: Instance of 'DocumentSnapshot'

Navigation from Main Page

class MyApp extends StatelessWidget {
  Widget _getScreenId() {
    return StreamBuilder<FirebaseUser>(
        stream: FirebaseAuth.instance.onAuthStateChanged,
        builder: (BuildContext context, snapshot) {
          if (snapshot.hasData) {
            print(snapshot.data);
            return Landing(userId: snapshot.data.uid);
          } else {
            return Login();
          }
        });
  }

Navigation to Feed screen from Landing

children: <Widget>[
          Home(),
          Feed(userId: widget.userId),
          Profile(),
        ],

Feed Screen

class Feed extends StatefulWidget {
  final String userId;

  Feed({this.userId});

  @override
  _FeedState createState() => _FeedState();
}

class _FeedState extends State<Feed> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.white,
        elevation: 1,
        title: Text('feed screen'),
      ),
      backgroundColor: Colors.white,
      body: FutureBuilder(
        future: usersRef.document(widget.userId).get(),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          print(snapshot.data['email']);
          return Padding(
            padding: const EdgeInsets.only(top: 70, left: 20, right: 20),
            child: Column(
              children: <Widget>[
                Text('MeandYou')
              ],
            ),
          );
        },
      ),
    );
  }
}

usersRef

final usersRef = _firestore.collection('users');

User Model

User(
      {this.id,
      this.email,
      this.password,
      this.fullname,
      this.username,
      this.mobile,
      this.profileImgUrl,
      this.bio});

  factory User.fromDoc(DocumentSnapshot doc) {
    return User(
      id: doc.documentID,
      email: doc['email'],
      fullname: doc['fullname'],
      username: doc['username'],
      mobile: doc['mobile'],
      profileImgUrl: doc['profileImgUrl'],
      bio: doc['bio'] ?? ''
    );
  }

I have tried several options and I would appreciate some help

1 answers

solution1
 4 ACCPTED  2019-12-16 20:31:33

Futures aren't always ready, they're asynchronous. I would insert at least a null check but here's a better way. It's best to check the ConnectionState of the Future to check if it's ready, then check to see whether or not the data is empty. Another bonus of using ConnectionState is the ability to check whether or not it's waiting for something. You can display a loading icon or something else then.

class _FeedState extends State<Feed> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.white,
        elevation: 1,
        title: Text('feed screen'),
      ),
      backgroundColor: Colors.white,
      body: FutureBuilder(
        future: usersRef.document(widget.userId).get(),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasData) {
              print(snapshot.data['email']);
              return Padding(
                padding: const EdgeInsets.only(top: 70, left: 20, right: 20),
                child: Column(
                  children: <Widget>[
                    Text('MeandYou')
                  ],
                ),
              );
            }
          }
        },
      ),
    );
  }
}

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