错误：正文可能正常完成，导致返回“null”，但返回类型是不可为空的类型

Question

I am building a flashChat app using FireBase.It is basically a chat app that can be used to send messages.And the messages are stored inside the FireBase collection.We have to retrieve and display the messages on the screen.

To display the contents(snapshots) I am using Streams. I am getting the following error

error: The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type. (body_might_complete_normally at [flashchat1] lib\screens\chat_screen.dart:80)

Here is my Code:-

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flashchat1/constants.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
class ChatScreen extends StatefulWidget {
  static String id='Chat_Screen';
  @override
  _ChatScreenState createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> {
  final _fireStore = FirebaseFirestore.instance;//an instance of fireBase store that stored data created
  final _auth = FirebaseAuth.instance;//instance/object of fireBase auth that authorizes users is created
  late User loggedInUser;//LoggedInUser is of type FireBase user(now changed to user)
  late String messageText;
  @override
  void initState()
  {
    super.initState();
    getCurrentUser();//calling the getCurrentUser
  }
  void getCurrentUser()
  async{
    try
    {
      final user= await _auth.currentUser;//get the current user id/name.Also currentUser return a future so make it async by adding await and async keywords
      if(user!=null)
      {
        loggedInUser=user ;//LoggedInUser = user contains email of the users
        print(loggedInUser.email);
      }

    }
    catch(e)
    {
      print(e);
    }
  }// Under collection there is documents.Inside documents there are fields like type ,values etc.These fields contain our information
    Future<void> messageStream()//Using a stream it becomes very easy .U just need to click once after you run the app .Then u will be done.
    async {//The snapShot here is FireBase's Query SnapShot
      await for(var snapshot in _fireStore.collection('messages').snapshots()){//make a variable snapshot to store the entire items of the collection in fireBase (Look at the fireBase console there is a collection called messages).This collection takes the snapshot of all the iteams (not literal snapshot .Think it like a snapShot)
        for(var message in snapshot.docs)//make a variable message to access the snapShot.docs .(docs stands for Documentation.Look at the fireBase console)
        print(message.data());
      }
    }
  void getMessages()//(The problem with this is that we need to keep clicking on the onPressed button every single time the new message is sent .So it is not convinient
  async {
    final messages = await _fireStore.collection('messages').get();//to retrieve the data from fire base we are creating a variable message
   messages.docs;//retreive the data from document section under the collection in firestore
    for(var message in messages.docs)//since it is a messages.docs is a list we need to loop through it
       {
        print(message.data());//print the data its messge.data()
     }
  }
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        leading: null,
        actions: <Widget>[
          IconButton(
              icon: Icon(Icons.close),
              onPressed: () {
                messageStream();
                //_auth.signOut();
                //Navigator.pop(context);
                //Implement logout functionality
              }),
        ],
        title: Text('⚡️Chat'),
        backgroundColor: Colors.lightBlueAccent,
      ),
      body: SafeArea(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            StreamBuilder(
              stream:_fireStore.collection('messages').snapshots(),
              builder: (context, AsyncSnapshot snapshot) {//This is Flutter's Async snapShot
                if(snapshot.hasData){//flutters async snapshot contains a query snapshot
                  final messages = snapshot.data.docs;
                  List<Text> messageWidgets = [];
                  for(var  message in messages)//Loop through the messages
                    {
                      final messageText = message.data['text'];//retrieve the data under the text field in message collection
                      final messageSender = message.data['Sender'];//retrieve the data under the Sender field in message collection
                      final messageWidget = Text('$messageText from $messageSender');
                      messageWidgets.add(messageWidget);
                      }
                      return Column(
                        children: messageWidgets,
                      );
                    }
                },
            ),
            Container(
              decoration: kMessageContainerDecoration,
              child: Row(
                crossAxisAlignment: CrossAxisAlignment.center,
                children: <Widget>[
                  Expanded(
                    child: TextField(
                      onChanged: (value) {
                        messageText=value;//Whatever you chat will be stored in the variable String variable messageText
                      },
                      decoration: kMessageTextFieldDecoration,
                    ),
                  ),
                  FlatButton(
                    onPressed: () {
                      _fireStore.collection('messages').add({
                        'text': messageText,//add the messages sent to fireStore under the messages object that we created manually
                        'Sender': loggedInUser.email,//add the current users email to the sender field
                      },);
                    },//goal is to send the data that we type here to the fireStore cloud
                    child: Text(
                      'Send',
                      style: kSendButtonTextStyle,
                    ),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
} 

Answer 1

You don't return anyting in your StreamBuilder when the if condition is not resolved...

Just add a else with, say... SizedBox.shrink() or whatever you need

Or return an empty Column