Null check operator cannot used on null value - firebase

Question

this is my chatscreen code
import 'package:flutter/material.dart';
import 'package:messenger/helperfunction/sharedpref_helper.dart';
import 'package:messenger/services/database.dart';
import 'package:random_string/random_string.dart';
class ChatScreen extends StatefulWidget {
final String chatWithusername, name;
ChatScreen(this.chatWithusername, this.name);
#override
_ChatScreenState createState() => _ChatScreenState();
}
class _ChatScreenState extends State<ChatScreen> {
String? chatRoomId, messageId = "";
String? myName, myProfilePic, myUserName, myEmail;
TextEditingController messageEditingController = TextEditingController();
getMyInfoFromSharedPreference() async {
myName = await SharedPreferenceHelper().getDisplayName();
myProfilePic = await SharedPreferenceHelper().getUserProfileUrl();
myUserName = await SharedPreferenceHelper().getUserName();
myEmail = await SharedPreferenceHelper().getUserEmail();
chatRoomId = getChatRoomIdByUsername(widget.chatWithusername, myUserName);
}
getChatRoomIdByUsername(String? a, String? b) {
if (a!.substring(0, 1).codeUnitAt(0) > b!.substring(0, 1).codeUnitAt(0)) {
return "$b\_$a";
} else {
return "$a\_$b";
}}
addMessage(bool sentClicked) {
if (messageEditingController.text != "") {
String message = messageEditingController.text;
var lastMessageTs = DateTime.now();
Map<String, dynamic> messageInfoMap = {
"message": message,
"sentBy": myUserName,
'ts': lastMessageTs,
"imgUrl": myProfilePic
};
//Gentrate the message id
if (messageId == "") {
messageId = randomAlphaNumeric(12);
}
DatabaseMethods()
.addMessage(chatRoomId!, messageId!, messageInfoMap)
.then((value) {
Map<String, dynamic>? lastMessageInfoMap = {
"lastmessage": message,
"lastMessageSendTs": lastMessageTs,
"lastMessageSendBy": myUserName
};
DatabaseMethods()
.updateLastmessageSend(chatRoomId!, lastMessageInfoMap);
if (sentClicked) {
messageEditingController.text = "";
messageId = "";
}
});
}
}
getAnfSentMessages() async {}
doThisOnlaunch() async {
await getMyInfoFromSharedPreference();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.name),
),
body: Container(
child: Stack(
children: [
Container(
alignment: Alignment.bottomCenter,
child: Container(
color: Colors.black.withOpacity(0.8),
padding: EdgeInsets.symmetric(
horizontal: 6,
vertical: 8,
),
child: Row(
children: [
Expanded(
child: TextField(
controller: messageEditingController,
onChanged: (value) {
addMessage(false);
},
style: TextStyle(color: Colors.white),
decoration: InputDecoration(
hintText: " Type a message",
hintStyle: TextStyle(
color: Colors.white.withOpacity(0.6))),
),
),
GestureDetector(
onTap: () {
addMessage(true);
},
child: Icon(Icons.send, color: Colors.white))
],
),
),
)
],
),
),
);
}
}
This is my DataBase;
import 'package:cloud_firestore/cloud_firestore.dart';
// ignore: unused_import
import 'package:firebase_core/firebase_core.dart';
class DatabaseMethods {
Future addUserInfoToDB(
String userId, Map<String, dynamic> userInfoMap) async {
return FirebaseFirestore.instance
.collection("users")
.doc(userId)
.set(userInfoMap);
}
Future<Stream<QuerySnapshot>> getUserByUserName(String username) async {
return FirebaseFirestore.instance
.collection("users")
.where("username", isEqualTo: username)
.snapshots();
}
Future addMessage(String chatRoomId, String messageId,
Map<String, dynamic> messageInfoMap) async {
return FirebaseFirestore.instance
.collection("chatRooms")
.doc(chatRoomId)
.collection("chats")
.doc(messageId)
.set(messageInfoMap);
}
updateLastmessageSend(
String chatRoomId, Map<String, dynamic> lastMessageInfoMap) {
return FirebaseFirestore.instance
.collection("chatRooms")
.doc(chatRoomId)
.update(lastMessageInfoMap);
}
createChatRoom(
String chatRoomId, Map<String, dynamic> chatRoomInfoMap) async {
final snapShot = await FirebaseFirestore.instance
.collection("chatrooms")
.doc(chatRoomId)
.get();
if (snapShot.exists) {
//chatroom already exits
return true;
} else {
//chatRoom does not exit:
return FirebaseFirestore.instance
.collection("chatrooms")
.doc(chatRoomId)
.set(chatRoomInfoMap);
}
}
}
when i run this code and start typing in textformfield, i am getting this following error:
'''
════════ Exception caught by widgets
═══════════════════════════════════════════
The following _CastError was thrown while calling onChanged:
Null check operator used on a null value
When the exception was thrown, this was the stack
#0 _ChatScreenState.addMessage
package:messenger/view/chatscreen.dart:54
#1 _ChatScreenState.build.<anonymous closure>
package:messenger/view/chatscreen.dart:103
#2 EditableTextState._formatAndSetValue
package:flutter/…/widgets/editable_text.dart:2298
#3 EditableTextState.updateEditingValue
package:flutter/…/widgets/editable_text.dart:1749
#4 TextInput._handleTextInputInvocation
package:flutter/…/services/text_input.dart:1351
'''
After typed, when i clicked sent icon i am getting error like this:
'''
════════ Exception caught by gesture
═══════════════════════════════════════════
Null check operator used on a null value
════════════════════════════════════════════════════════════════════════
'''
can anyone help to solve this issue

Answer 1

After digging into your code I can see that this function is never called
doThisOnlaunch() async {
await getMyInfoFromSharedPreference();
}
So the getMyInfoFromSharedPreference() function is never called
So chatRoomId is not initialized and it will always be null Because of this code
String? chatRoomId, messageId = "";
And in this method you are using the ! on chatRoomId which is null.
DatabaseMethods()
.addMessage(chatRoomId!, messageId!, messageInfoMap).then((value) {...
So this is what causes the error.
Solution
Call tis method on initState like
String? chatRoomId, messageId = "";
String? myName, myProfilePic, myUserName, myEmail;
TextEditingController messageEditingController = TextEditingController();
#override
void initState() {
super.initState();
doThisOnlaunch();
}
.
.
.
or just initiate chatRoomId with a value and declare them as not nullable String
String chatRoomId="", messageId = "";
Note that this second solution will just stop the error but your code will not function properly

Answer 2

Somewhere you used ! on a variable and that variable is null. Check the error where it is.

Answer 3

getChatRoomIdByUsername(String? a, String? b) {
if (a!.substring(0, 1).codeUnitAt(0) > b!.substring(0, 1).codeUnitAt(0)) {
return "$b\_$a";
} else {
return "$a\_$b";
}}
Why are you allowing nullable variables if the function depends on this?
getChatRoomIdByUsername({required String a,required String b}) {
if (a.substring(0, 1).codeUnitAt(0) > b.substring(0, 1).codeUnitAt(0)) {
return "$b\_$a";
} else {
return "$a\_$b";
}
}