Unhandled Exception: type 'Null' is not a subtype of type 'String' in type cast flutter - flutter

Question

I'm trying to make a simple messaging application while using firebase and flutter, everything was working fine until I came to the message screen, but on this screen, I get the error that I will specify below. Do you know the solution for this?
Unhandled Exception: type 'Null' is not a subtype of type 'String' in type cast
I could not understand where I got the error, when I click on any user on the page where I listed the users, it gives this error directly.
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';
import 'package:ourchat/pallete.dart';
import 'package:ourchat/shared/utils.dart';
class ChatDetailPage extends StatefulWidget {
final String user;
const ChatDetailPage({
Key? key,
required this.user,
}) : super(key: key);
#override
_ChatDetailPageState createState() => _ChatDetailPageState();
}
class _ChatDetailPageState extends State<ChatDetailPage> {
TextEditingController messageController = TextEditingController();
final currentUserId = FirebaseAuth.instance.currentUser?.uid;
CollectionReference chats = FirebaseFirestore.instance.collection('chats');
var chatDocId;
var userData = {};
bool isLoading = false;
#override
void initState() {
super.initState();
getData();
checkUser();
}
void checkUser() async {
await chats
.where('users', isEqualTo: {userData['uid']: null, currentUserId: null})
.limit(1)
.get()
.then(
(QuerySnapshot querySnapshot) async {
if (querySnapshot.docs.isNotEmpty) {
setState(() {
chatDocId = querySnapshot.docs.single.id;
});
print(chatDocId);
} else {
await chats.add({
'users': {currentUserId: null, widget.user: null},
'names': {
currentUserId: FirebaseAuth.instance.currentUser?.displayName,
userData['uid']: userData['username']
}
}).then((value) => {chatDocId = value});
}
},
)
.catchError((error) {});
}
getData() async {
setState(() {
isLoading = true;
});
try {
var userSnap = await FirebaseFirestore.instance
.collection('users')
.doc(widget.user)
.get();
userData = userSnap.data()!;
print(userData);
setState(() {});
} catch (e) {
showSnackBar(
context,
e.toString(),
);
}
setState(() {
isLoading = false;
});
}
void sendMessage(String msg) {
if (msg == '') return;
chats.doc(chatDocId).collection('messages').add({
'createdOn': FieldValue.serverTimestamp(),
'uid': currentUserId,
'friendName': userData['username'],
'msg': msg
}).then((value) {
messageController.text = '';
});
}
bool isSender(String friend) {
return friend == currentUserId;
}
Alignment getAlignment(friend) {
if (friend == currentUserId) {
return Alignment.topRight;
}
return Alignment.topLeft;
}
#override
Widget build(BuildContext context) {
return isLoading
? const Center(
child: CircularProgressIndicator(),
)
: StreamBuilder<QuerySnapshot>(
stream: FirebaseFirestore.instance
.collection('users')
.doc(chatDocId)
.collection('messages')
.orderBy('createdOn', descending: true)
.snapshots(),
builder: (context, snapshot) {
if (snapshot.hasError) {
return Center(
child: Text("Bir şeyler yanlış gitti..."),
);
}
if (snapshot.connectionState == ConnectionState.waiting) {
return Center(
child: CircularProgressIndicator(
color: kBlue,
),
);
}
if (snapshot.hasData) {
var data;
return Scaffold(
backgroundColor: Colors.black,
appBar: AppBar(
elevation: 0,
automaticallyImplyLeading: false,
backgroundColor: Colors.black,
flexibleSpace: SafeArea(
child: Container(
padding: EdgeInsets.only(right: 16),
child: Row(
children: <Widget>[
IconButton(
onPressed: () {
Navigator.pop(context);
},
icon: ImageIcon(
AssetImage("assets/icons/backicon.png"),
color: Colors.white,
),
),
SizedBox(
width: 2,
),
CircleAvatar(
backgroundImage:
NetworkImage(userData['photoUrl'].toString()),
maxRadius: 20,
),
SizedBox(
width: 12,
),
Expanded(
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
userData['username'].toString(),
style: TextStyle(
fontSize: 16,
fontWeight: FontWeight.w600,
color: Colors.white),
),
SizedBox(
height: 6,
),
Text(
userData['bio'].toString(),
style: TextStyle(
color: Colors.grey.shade600,
fontSize: 13,
),
),
],
),
),
Icon(
Icons.settings,
color: Colors.white,
),
],
),
),
),
),
body: Stack(
children: <Widget>[
ListView(
reverse: true,
children: snapshot.data!.docs.map(
(DocumentSnapshot documentSnapshot) {
data = documentSnapshot.data()!;
print(documentSnapshot.toString());
print(data['msg']);
return Padding(
padding:
const EdgeInsets.symmetric(horizontal: 8.0),
child: 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(
children: [
Row(
mainAxisAlignment:
MainAxisAlignment.start,
children: [
Text(data['msg'],
style: TextStyle(
color: isSender(data['uid']
.toString())
? Colors.white
: Colors.black),
maxLines: 100,
overflow: TextOverflow.ellipsis)
],
),
Row(
mainAxisAlignment:
MainAxisAlignment.end,
children: [
Text(
data['createdOn'] == null
? DateTime.now().toString()
: data['createdOn']
.toDate()
.toString(),
style: TextStyle(
fontSize: 10,
color: isSender(
data['uid'].toString())
? Colors.white
: Colors.black),
)
],
)
],
),
),
),
);
},
).toList(),
),
Align(
alignment: Alignment.bottomLeft,
child: Container(
padding:
EdgeInsets.only(left: 10, bottom: 10, top: 10),
height: 60,
width: double.infinity,
color: Colors.black,
child: Row(
children: <Widget>[
GestureDetector(
onTap: () {},
child: Container(
height: 40,
width: 40,
decoration: BoxDecoration(
color: kBlue,
borderRadius: BorderRadius.circular(30),
),
child: Icon(
Icons.add,
color: Colors.white,
size: 26,
),
),
),
SizedBox(
width: 15,
),
Expanded(
child: TextField(
controller: messageController,
style: TextStyle(color: Colors.white),
decoration: InputDecoration(
hintText: "Lütfen mesaj yazınız.",
labelStyle:
TextStyle(color: Colors.white),
hintStyle: TextStyle(color: Colors.white),
border: InputBorder.none),
),
),
SizedBox(
width: 15,
),
GestureDetector(
onTap: () {
sendMessage(messageController.text);
},
child: Container(
height: 40,
width: 40,
decoration: BoxDecoration(
color: kBlue,
borderRadius: BorderRadius.circular(30),
),
child: Icon(
Icons.send,
color: Colors.white,
size: 20,
),
),
),
],
),
),
),
],
),
);
}
return null!;
},
);
}
}