Null Check Operator Used On a Null Value? GetStorage in flutter give me this error - flutter

Question

I am using GetStorage package in Flutter to check if the current user is login or not but it gives the error.
For example, when a user does not login it shows the null error and when I login it then it is filn, check if you can.
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:mobihub_2/Screens/main_login.dart';
import 'package:mobihub_2/Screens/navigationscreens.dart/add.dart';
import 'navigationscreens.dart/adds.dart';
import 'navigationscreens.dart/chat.dart';
import 'navigationscreens.dart/home2.dart';
import 'navigationscreens.dart/person.dart';
class Home extends StatefulWidget {
const Home({Key? key}) : super(key: key);
#override
// ignore: library_private_types_in_public_api
_HomeState createState() => _HomeState();
}
class _HomeState extends State<Home> {
final auth = FirebaseAuth.instance;
int currentTab = 0;
final List<Widget> screens = const [
Home2(),
Chat(),
Youradds(),
Account(),
];
final PageStorageBucket bucket = PageStorageBucket();
Widget currentScreen = const Home2();
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.grey.shade100,
body: PageStorage(
bucket: bucket,
child: currentScreen,
),
floatingActionButton: FloatingActionButton(
backgroundColor: const Color(0xFFFFDC3D),
elevation: 2.5,
onPressed: () {
if(FirebaseAuth.instance.currentUser != null){
Navigator.push(context,
MaterialPageRoute(builder: (context) => const AddItems()));
}else{
Navigator.push(context,
MaterialPageRoute(builder: (context) => MainLogin()));
}
},
child: const Icon(
Icons.add,
color: Colors.black,
),
),
floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
bottomNavigationBar: BottomAppBar(
color: Colors.white,
shape: const CircularNotchedRectangle(),
notchMargin: 10,
child: SizedBox(
height: 60,
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Row(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
MaterialButton(
highlightElevation: 0.0,
highlightColor: Colors.white,
hoverColor: Colors.white,
hoverElevation: 0.0,
minWidth: 40,
onPressed: () {
setState(() {
currentScreen = const Home2();
currentTab = 0;
});
},
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(
Icons.home,
color: currentTab == 0
? const Color(0xFFFFDC3D)
: Colors.black,
size: 25,
),
Text(
'',
style: TextStyle(
color: currentTab == 0
? const Color(0xFFFFDC3D)
: Colors.white),
)
],
),
),
MaterialButton(
highlightElevation: 0.0,
highlightColor: Colors.white,
hoverColor: Colors.white,
hoverElevation: 0.0,
minWidth: 40,
onPressed: () {
setState(() {
if(FirebaseAuth.instance.currentUser != null){
currentScreen = const Chat();
currentTab = 1;
}else{
Navigator.push(context,
MaterialPageRoute(builder: (context) => MainLogin()));
}
});
},
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(
Icons.chat,
color: currentTab == 1
? const Color(0xFFFFDC3D)
: Colors.black,
size: 25,
),
Text(
'',
style: TextStyle(
color: currentTab == 1
? const Color(0xFFFFDC3D)
: Colors.white),
)
],
),
)
],
),
Row(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
MaterialButton(
highlightElevation: 0.0,
highlightColor: Colors.white,
hoverColor: Colors.white,
hoverElevation: 0.0,
minWidth: 40,
onPressed: () {
setState(() {
if(FirebaseAuth.instance.currentUser != null){
currentScreen = const Youradds();
currentTab = 2;
}else{
Navigator.push(context,
MaterialPageRoute(builder: (context) => MainLogin()));
}
});
},
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(
Icons.favorite,
color: currentTab == 2
? const Color(0xFFFFDC3D)
: Colors.black,
size: 25,
),
Text(
'',
style: TextStyle(
color: currentTab == 2
? const Color(0xFFFFDC3D)
: Colors.white),
)
],
),
),
MaterialButton(
highlightElevation: 0.0,
highlightColor: Colors.white,
hoverColor: Colors.white,
hoverElevation: 0.0,
autofocus: false,
minWidth: 40,
onPressed: () {
setState(() {
if(FirebaseAuth.instance.currentUser != null){
currentScreen = Account();
currentTab = 3;
}else{
Navigator.push(context,
MaterialPageRoute(builder: (context) => MainLogin()));
}
});
},
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(
Icons.person,
color: currentTab == 3
? const Color(0xFFFFDC3D)
: Colors.black,
size: 25,
),
Text(
'',
style: TextStyle(
color: currentTab == 3
? const Color(0xFFFFDC3D)
: Colors.white),
)
],
),
)
],
)
],
),
),
),
);
}
}
I need help if anyone expert in Flutter checks my code and gives me the right answer. Thank you in advance.