如何修复未处理的异常：NoSuchMethodError：在 null flutter firestore 上调用了方法“[]”

Question

I am trying to implement method to delete comment (if it is my own comment, or comment that was posted under my post), report user and/or block user (if I find their comments offensive or whatever). However, my method that calls dialog menu:

CommentModel com;

handleComments(BuildContext context) async {
DocumentSnapshot snap = await 
commentRef.doc(widget.post.postId).collection('comments')
.doc().get();
com = CommentModel.fromJson(snap.data());
var commentUserId = com.userId;
bool isMe = widget.post.userId == commentUserId;
bool isComUser = currentUserId() == commentUserId;
if (isMe || isComUser)
  showDialog(
    context: context,
    builder: (context) {
      return SimpleDialog(
        backgroundColor: Colors.red,
        shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(15.0)),
        children: [
          SimpleDialogOption(
            onPressed: () {
              deleteComment(context);
            },
            child: Text(Languages
                .of(context)
                .deleteComment, style: TextStyle(color: Colors.white),),
          ),
        ],
      );
    });
else
  showDialog(
      context: context,
      builder: (context) {
        return SimpleDialog(
            backgroundColor: Colors.red,
            shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(15.0)),
            children: [
SimpleDialogOption(
onPressed: () {
blockUser(context);
},
child: Text(Languages
    .of(context)
    .blockUser, style: TextStyle(color: Colors.white),),
),
Divider(),
SimpleDialogOption(
onPressed: () {
reportUser();
},
child: Text(Languages
    .of(context)
.reportUser, style: TextStyle(color: Colors.white),),
),
        ]);
      });

}

gives error:

Unhandled Exception: NoSuchMethodError: The method '[]' was called on null.
E/flutter (13165): Receiver: null
E/flutter (13165): Tried calling: []("username")
E/flutter (13165): #0      Object.noSuchMethod (dart:core- 
patch/object_patch.dart:63:5)
E/flutter (13165): #1      new CommentModel.fromJson 
(package:blahblah/comments.dart:17:20)
E/flutter (13165): #2      _CommentsState.handleComments 
(package:blahblah/comment.dart:438:19)
E/flutter (13165): <asynchronous suspension>`

Even though adding comments to firestore is no problem, and CommentModel has all the fields in the doc (please, see pic)

Where could be the issue? I went through several similar posts, but could not find the solution. Thank you for your help!

here is the CommentModel class:

class CommentModel {
String username, comment, userDp, userId, docId;
Timestamp timestamp;

CommentModel({
this.username,
this.comment,
this.timestamp,
this.userDp,
this.userId,
this.docId
});

CommentModel.fromJson(Map<String, dynamic> json) {
username = json['username'];
comment = json['comment'];
timestamp = json['timestamp'];
userDp = json['userDp'];
userId = json['userId'];
docId = json['docId'];
}

Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['username'] = this.username;
data['comment'] = this.comment;
data['timestamp'] = this.timestamp;
data['userDp'] = this.userDp;
data['userId'] = this.userId;
data['docId'] = this.docId;
return data;
}

}

Answer 1

From two related threads , this issue originates from Firestore not being able to find the document you are requesting. I looked at your attached image and it shows multiple documents inside the comments subcollection. It appears that your second doc() call in your request is missing the required documentID argument :

DocumentSnapshot snap = await commentRef.doc(widget.post.postId).collection('comments').doc().get()

I tested a similar query in a test Flutter app and it also returned null in my case. The issue on my end was resolved after providing a documentID on all doc() method calls. If you would like to get all documents from the comments collection you can make use of this documentation page for FlutterFire.