无法无条件调用方法“[]”，因为接收器可以为“null”。尝试使调用有条件（使用'？.'）或向目标（'！'）.
我得到这个错误，而试图从Firebase云数据流我不知道该怎么办

StreamBuilder<QuerySnapshot>(
              stream: _firestore.collection('messages').snapshots(),
              builder: (context, snapshot) {
                List<Text> messageWidgets = [];
                if (snapshot.hasData) {
                  final messages = snapshot.data?.docs;

                  for (var message in messages!) {
                    final messageData = message.data();
                    final messageText = messageData['text'];
                    final messageSender = messageData['sender'];
                    final messageWidget =
                        Text('$messageSender said $messageText');
                    messageWidgets.add(messageWidget);
                  }
                }
                return Column(
                  children: messageWidgets,
                );
              },
            ),

这两行的误差

final messageText = messageData['text'];
 final messageSender = messageData['sender'];

尝试从firbase云获取和传输数据