The method 'substring' was called on null. Receiver: null Tried calling: substring(0, 1) - flutter

Question

I'm programming a chat application using flutter and firebase. Whenever I click on "Message" button to go to Chat Room screen following error appears:
The method 'substring' was called on null.
Receiver: null
Tried calling: substring(0, 1)
Here's the code of search view. Currently , I'm working to get ChatRoomID by a function to implement chatroom screen but it is showing an error that the substring was null.
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:howdy_do/View/chatroomscreen.dart';
import 'package:howdy_do/View/conversation_screen.dart';
import 'package:howdy_do/helper/constants.dart';
import 'package:howdy_do/helper/helperfunctions.dart';
import 'package:howdy_do/services/database.dart';
import 'package:howdy_do/widgets/widget.dart';
class Search extends StatefulWidget {
#override
_SearchState createState() => _SearchState();
}
class _SearchState extends State<Search> {
DatabaseMethods databaseMethods = new DatabaseMethods();
TextEditingController searchTextEditingController = new TextEditingController();
QuerySnapshot searchResultSnapshot;
Widget searchList(){
return searchResultSnapshot != null ? ListView.builder(
itemCount: searchResultSnapshot.docs.length ,
shrinkWrap: true,
itemBuilder: (context, index) {
return SearchTile(
userName: searchResultSnapshot.docs[index].data()["name"],
userEmail: searchResultSnapshot.docs[index].data()["email"],
);
}) : Container();
}
initiateSearch(){
DatabaseMethods().getUserByUsername(searchTextEditingController.text)
.then((val){
setState(() {
searchResultSnapshot = val;
print("$searchResultSnapshot");
});
});}
Widget SearchTile( {String userName, String userEmail}){
return Container(
padding: EdgeInsets.symmetric(horizontal: 20, vertical: 16),
child: Row(
children: [
Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Text(userName, style: simpleTextStyle(),),
Text(userEmail, style: simpleTextStyle(),)
],
),
Spacer(),
GestureDetector(
onTap:() {
createChatroomAndStartConversation( **// Error appears here**
userName: userName
);
},
child: Container(
padding: EdgeInsets.symmetric(horizontal: 16, vertical: 8),
child: Text("Message", style: mediumTextStyle(),),
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(24),
gradient: LinearGradient(
colors:[
const Color(0xffF7A1A0),
const Color(0xffF8A3A2)
]
),
)
),
)
],
),
);
}
createChatroomAndStartConversation({ String userName}){
print("${Constants.myName}");
if(userName != Constants.myName) {
List<String> users = [userName,Constants.myName];
String chatRoomId = getChatRoomId(userName,Constants.myName); **// Error appears here**
Map<String,dynamic> chatRoomMap= {
"users": users,
"chatRoomId" : chatRoomId
};
DatabaseMethods().createChatRoom(chatRoomId, chatRoomMap) ;
Navigator.push(context, MaterialPageRoute(
builder: (context) => ConversationScreen(chatRoomId)
));
}else {
print("You can't send text to yourself");
}
}
#override
void initState() {
super.initState();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: appBarMain(context),
body:Container(
child: Column(
children: [
Container(
padding: EdgeInsets.symmetric(horizontal: 15, vertical: 16),
child: Row(
children: [
Expanded(
child: TextField(
controller: searchTextEditingController,
style: TextStyle(
color: const Color(0xffa1a0ba),
fontFamily: 'Poppins',
),
decoration: InputDecoration(
filled: true,
fillColor: const Color(0xffffffff),
border: UnderlineInputBorder(
borderRadius:BorderRadius.circular(30.0)),
hintText: "search username...",
hintStyle: TextStyle(
color: Colors.black12,
fontFamily: 'Poppins',
),
),
)
),
GestureDetector(
onTap: (){
initiateSearch();
},
child: Container(
height: 40,
width: 40,
padding: EdgeInsets.all(8),
child:Image.asset("assets/images/search_white.png")
) ),
],
),
),
searchList() ],), ), ); } }
getChatRoomId(String a, String b) {
if(a.substring(0, 1).codeUnitAt(0)> b.substring(0, 1).codeUnitAt(0)){ **//And error appears here too**
return "$b\_$a";
} else {
return "$a\_$b";
}
}

Answer 1

What's happening is that you are passing getChatRoomId a null value so when you call the substring method it gives you an error because it cannot process a null value. Most probably you are unable to read data from firebase. I suggest you try to print the chatRoodID to make sure you are not getting a null value.