Flutter error : 'isDocument()': is not true - flutter

Question

I am making a chat detail page using flutter. However, I am getting this error in the stream part. I don't have much experience with flutter. I couldn't solve it for 2 days.
Error :
Exception has occurred.
_AssertionError ('package:cloud_firestore_platform_interface/src/internal/pointer.dart': Failed assertion: line 56 pos 12: 'isDocument()': is not true.)
Code :
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:flutter_chat_bubble/bubble_type.dart';
import 'package:flutter_chat_bubble/chat_bubble.dart';
import 'package:flutter_chat_bubble/clippers/chat_bubble_clipper_6.dart';
class Chat extends StatefulWidget {
Chat({super.key, required this.friendUid, required this.friendName});
final String friendUid;
final String friendName;
#override
State<Chat> createState() => _ChatState(friendUid, friendName);
}
class _ChatState extends State<Chat> {
CollectionReference chats = FirebaseFirestore.instance.collection("chats");
final friendUid;
final friendName;
final currentUserUid = FirebaseAuth.instance.currentUser!.uid;
var chatDocId;
var _controller = TextEditingController();
_ChatState(this.friendName, this.friendUid);
#override
void initState() {
chats
.where("users", isEqualTo: {friendUid: null, currentUserUid: null})
.limit(1)
.get()
.then(
(QuerySnapshot snapshotx) {
if (snapshotx.docs.isNotEmpty) {
chatDocId = snapshotx.docs.single.id;
} else {
chats.add({
"users": {currentUserUid: null, friendUid: null},
}).then((value) {
chatDocId = value;
});
}
},
)
.catchError(() {});
super.initState();
}
void sendMessage(String msg) {
if (msg == "") return;
chats.doc(chatDocId.toString()).collection("messages").add({
"createdOn": FieldValue.serverTimestamp(),
"uid": currentUserUid,
"msg": msg
}).then((value) {
_controller.text = "";
});
}
bool isSender(String friend) {
return friend == currentUserUid;
}
Alignment getAlignment(String friend) {
if (friend == currentUserUid) {
return Alignment.topRight;
}
return Alignment.topLeft;
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text("chat")),
body: Padding(
padding: const EdgeInsets.all(8.0),
child: Column(
mainAxisSize: MainAxisSize.max,
children: [
Expanded(
child: StreamBuilder<QuerySnapshot>(
stream: chats
.doc(chatDocId.toString())
.collection("messages") ==> the error part
.orderBy("createdOn", descending: true)
.snapshots(),
builder: (BuildContext context,
AsyncSnapshot<QuerySnapshot> snapshot) {
if (snapshot.hasError) {
return const Center(
child: Text("Error"),
);
}
if (snapshot.connectionState ==
ConnectionState.waiting) {
return Center(child: CircularProgressIndicator());
}
if (snapshot.hasData) {
return ListView(
reverse: true,
children: snapshot.data!.docs
.map((DocumentSnapshot documentSnapshot) {
Map<String, dynamic> data = documentSnapshot
.data() as Map<String, dynamic>;
return ChatBubble(
clipper: ChatBubbleClipper6(
nipSize: 0,
radius: 0,
type: isSender(data["uid"].toString())
? BubbleType.sendBubble
: BubbleType.receiverBubble),
alignment:
getAlignment(data["uid"].toString()),
margin: EdgeInsets.only(top: 20),
backGroundColor:
isSender(data["uid"].toString())
? Color(0xFF08C187)
: Color(0xffE7E7ED),
child: Container(
constraints: BoxConstraints(
maxWidth: MediaQuery.of(context)
.size
.width *
0.7),
child: Column(
mainAxisSize: MainAxisSize.max,
children: [
Row(
mainAxisSize: MainAxisSize.max,
mainAxisAlignment:
MainAxisAlignment.start,
children: [
Text(
data["msg"],
style: TextStyle(
color: isSender(
data["uid"]
.toString())
? Colors.white
: Colors.black),
maxLines: 100,
overflow:
TextOverflow.ellipsis,
)
],
)
],
)),
);
}).toList());
}
return Container(
width: 300,
height: 300,
color: Colors.grey,
);
})),
Row(
mainAxisSize: MainAxisSize.max,
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Expanded(
child: TextField(
controller: _controller,
)),
IconButton(
onPressed: () {
sendMessage(_controller.text);
},
icon: Icon(Icons.send))
],
)
],
),
));
}
}
Can you help me. Thank you.