I'm building a Messenger app on Flutter using Firebase and when I click to chat with another user - I get this exception before the previous messages are loaded.

  ======== Exception caught by widgets library =======================================================
The following LateError was thrown building ChatRoom(dirty, state: _ChatRoomState#be2bb):
LateInitializationError: Field 'chatRoomsStream' has not been initialized.

The relevant error-causing widget was: 
  ChatRoom file:///D:/Android/chat_app/lib/main.dart:47:55
When the exception was thrown, this was the stack: 
#0      _ChatRoomState.chatRoomsStream (package:chat_app/views/chatRoomsScreen.dart)
#1      _ChatRoomState.chatRoomList (package:chat_app/views/chatRoomsScreen.dart:29:15)
#2      _ChatRoomState.build (package:chat_app/views/chatRoomsScreen.dart:83:15)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4691:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4574:15)
...
====================================================================================================

This is the chatRoomsScreen.dart page that is causing the issue, this is the page that is loaded every time a user wants to chat with another user:

    import 'package:chat_app/helper/authenticate.dart';
import 'package:chat_app/helper/constants.dart';
import 'package:chat_app/helper/helperfunctions.dart';
import 'package:chat_app/services/auth.dart';
import 'package:chat_app/services/database.dart';
import 'package:chat_app/views/conversationScreen.dart';
import 'package:chat_app/views/search.dart';
import 'package:chat_app/views/signin.dart';
import 'package:chat_app/widgets/widgets.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';

class ChatRoom extends StatefulWidget {
  const ChatRoom({Key? key}) : super(key: key);

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

class _ChatRoomState extends State<ChatRoom> {

  AuthMethods authMethods = new AuthMethods();
  DatabaseMethods databaseMethods = new DatabaseMethods();

  late Stream<QuerySnapshot> chatRoomsStream;

  Widget chatRoomList() {
    return StreamBuilder(
      stream: chatRoomsStream,
      builder: (context, snapshot) {
        return snapshot.hasData ? ListView.builder(
          itemCount: (snapshot.data as QuerySnapshot).docs.length,
          itemBuilder: (context, index) {
            return ChatRoomsTile(
                (snapshot.data as QuerySnapshot).docs[index]["chatroomid"]
                    .toString().replaceAll("_", "")
                    .replaceAll(Constants.myName, ""),
                (snapshot.data as QuerySnapshot).docs[index]["chatroomid"]
            );
          },
        ) : Container();
      },
    );
  }

  @override
  void initState() {
    getUserInfo();
    super.initState();
  }

  getUserInfo() async {
    Constants.myName = (await HelperFunctions.getUserNameSharedPreference())!;
    databaseMethods.getChatRooms(Constants.myName).then((value) {
      setState(() {
        chatRoomsStream = value;
      });
    });
    setState(() {

    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
              title: Image.asset("assets/images/logo.png", height: 50.0),

        actions: [
          GestureDetector(
            onTap: () {
              authMethods.signOut();
              Navigator.pushReplacement(context,
              MaterialPageRoute(builder: (context) => Authenticate()
              ));
            },
            child: Container(
              padding: EdgeInsets.symmetric(horizontal: 16),
                child: Icon(Icons.exit_to_app)),
          ),
        ],),
        body: chatRoomList(),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            Navigator.pushReplacement(context, MaterialPageRoute(
              builder: (context) => SearchScreen()
            ));
          },
          child: Icon(Icons.search),
        ),
    );
  }
}

class ChatRoomsTile extends StatelessWidget {
  late final String userName;
  late final String chatRoomId;

  ChatRoomsTile(this.userName,this.chatRoomId);
  
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        Navigator.push(context, MaterialPageRoute(
          builder: (context) => ConversationScreen(chatRoomId: chatRoomId)
        ));
      },
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24, vertical: 20),
        child: Row(
          children: [
            Container(
              height: 40,
              width: 40,
              alignment: Alignment.center,
              decoration: BoxDecoration(
                  color: Colors.amber,
                  borderRadius: BorderRadius.circular(30)),

              child: Text(userName.substring(0, 1),
                  textAlign: TextAlign.center,
                  style: TextStyle(
                      color: Colors.white,
                      fontSize: 16,
                      fontFamily: 'OverpassRegular',
                      fontWeight: FontWeight.w300)),
            ),
              SizedBox(width: 12,),
              Text(userName, style: mediumTextStyle(),),
          ],
        ),
      ),
    );
  }
}

What am I doing wrong with chatRoomsStream? I'm initializing it as early as I can and to me, it's done as it should be. Perhaps one of you can point out why I'm getting this exception every time this page is loaded - help would be greatly appreciated, thanks.

CodePudding user response：

For your kind information, Using late keyword means that the instant will be initialized when you use it for the first time. Like:

late int value = 5;
late int value; // you got the error 

For more about sound null safety

More about lazy initialization

N.B: In your case, you can remove late, otherwise initialize it.

CodePudding user response：

instead of late Stream<QuerySnapshot> chatRoomsStream;

use Stream<QuerySnapshot>? chatRoomsStream;

and when you call the chatRoomStream call it as chatRoomStream! this is null safety know about null safety