我们这里有一个屏幕应该从firestore云获取数据，但抛出这个错误，即使我确保检查每一个可能的空值，但没有解决这个错误。
看起来我遗漏了一些找不到的内容（值）错误可能位于ShowMessages类中
下面是代码：

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';

class GroupScreen extends StatelessWidget {
  final String groupUID;
  final List  members;
  final bool admin;
  final String title;
  final String username;

   const GroupScreen({
    super.key,
    required this.groupUID,
    required this.members,
    required this.admin,
    required this.title,
     required this.username
});

  @override
  Widget build(BuildContext context) {
    TextEditingController _msg = TextEditingController();
    return Scaffold(
      backgroundColor: Colors.deepPurple[50],
      appBar: AppBar(
        title: Text(title),
        backgroundColor: Colors.deepPurple[300],),
      body:  Column(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
             crossAxisAlignment: CrossAxisAlignment.stretch,
             children: [
               Text('Messages'),
                ShowMessages(groupUID: groupUID,),
                Row(
                  children: [

                    Expanded(
                        child: Container(
                          decoration: BoxDecoration(
                            border: Border(
                              top: BorderSide(color: Colors.deepPurpleAccent),

                            )
                          ),
                          child: TextField(

                            controller: _msg,
                            decoration: InputDecoration(hintText: 'Enter Message...',border: InputBorder.none),
                          ),
                        )
                    ),
                    IconButton(onPressed: (){
                      if(!_msg.text.isEmpty) {
                        FirebaseFirestore.instance.collection("users").
                            doc(FirebaseAuth.instance.currentUser!.uid).
                            collection("groups").
                                doc(groupUID).
                                  collection("chat").
                                    doc().
                                      set({
                                    "msg": _msg.text,
                                    "user":username

                                        });
                        _msg.clear();
                      }
                    }, icon: Icon(Icons.send, color: Colors.deepPurple,))
                  ],
                ),

             ],
            ),
          );
  }
  }

class ShowMessages extends StatelessWidget {
  final String groupUID;
  const ShowMessages({
    super.key,
    required this.groupUID
});

  @override
  Widget build(BuildContext context) {
    return StreamBuilder(

    stream: FirebaseFirestore.instance.collection("users").doc(FirebaseAuth.instance.currentUser!.uid).collection("groups").doc(groupUID).collection("chat").doc().snapshots(),
    builder: (context,snapshot){
      if (snapshot.hasData){
        return ListView.builder(
          shrinkWrap: true,
          primary: true,
          itemCount: snapshot.data!.data()!.length != null ? snapshot.data!.data()!.length : 0,
          itemBuilder: (context,index){
            QueryDocumentSnapshot value =  snapshot.data!.data()![index] != null ? snapshot.data!.data()![index] : null ;
            return ListTile(
              title:  Text(value != null ? value['msg'] : ''),
            );
          },
        );
      }
      return Text('');
    } ,
    );
  }
}