The method '[]' was called on null. Receiver: null Tried calling: []("displayName")

Question

This error is caused by what? The other files have the same code format, but there are no errors.

I use displayName as the document name.

Error

The method '[]' was called on null.

Receiver: null

Tried calling: [] ("displayName")

final DocumentSnapshot data;

    body: StreamBuilder<QuerySnapshot>(
              stream: Firestore.instance
                  .collection('users')
                  .document(widget.data['displayName'])
                  .collection('chat')
                  .snapshots(),
              builder: (context, snapshot) {
                if (!snapshot.hasData) return LinearProgressIndicator();
                return Container();
              })

DB:

static Future<void> sendChatToFirebase(
    String chatID,
    String chatContent,
    FirebaseUser currentUser,
    DocumentSnapshot data
  ) async {
    Firestore.instance
    .collection('users')
    .document(data.documentID)
    .collection('chat')
    .document(chatID)
    .setData({
      'chatID': chatID,
      'chatTimeStamp': DateTime.now().millisecondsSinceEpoch,
      'chatContent': chatContent,
      'displayName': currentUser.displayName,
      'photoUrl': currentUser.photoUrl,
    });
  }

Answer 1

The error message means, that you are trying to access ['displayName'] from a null value. Somehow your widget.data is null.

It's basically the same as calling null['displayName']

Map<String, dynamic> test; // NULL, but expected as Map / Json / {}

print(test["displayName"]); // throws an error, because what was actually called is null["displayName"]

Answer 2

The index [] operator on the Map class returns null if the key isn't present. This implies that the return type of that operator must be nullable. See null safety . This means that your widget.data['displayName'] may return null . To fix your issue you need to check that widget.data['displayName] is not null, if so then you can use the ! to say that widget.data['displayName] is not null.

widget.data['displayName']!