Flutter SharedPreferences non-nullable instance fields - flutter

Question

I have a problem when I want to using the shared preferences to store and display the value. On the code, I would need to create the public class for the bellow function to use.
However it would return the follow error:
error: Non-nullable instance field 'sharedPreferences' must be initialized. (not_initialized_non_nullable_instance_field at [login] lib/main.dart:32)
SharedPreferences sharedPreferences; //line:32
Here is my full code.
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:http/http.dart' as http;
import 'package:login/login_page.dart';
import 'package:shared_preferences/shared_preferences.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: "Code Land",
debugShowCheckedModeBanner: false,
home: MyHomePage(),
theme: ThemeData(
accentColor: Colors.white70
),
);
}
}
class MyHomePage extends StatefulWidget {
#override
createState() => MyHomePageState();
}
class MyHomePageState extends State<MyHomePage> {
SharedPreferences sharedPreferences;
#override
void initState() {
super.initState();
checkLoginStatus();
}
checkLoginStatus() async {
sharedPreferences = await SharedPreferences.getInstance();
if(sharedPreferences.getString("token") == null) {
Navigator.of(context).pushAndRemoveUntil(MaterialPageRoute(builder: (BuildContext context) => LoginPage()), (Route<dynamic> route) => false);
}
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Code Land", style: TextStyle(color: Colors.white)),
actions: <Widget>[
FlatButton(
onPressed: () {
sharedPreferences.clear();
sharedPreferences.commit();
Navigator.of(context).pushAndRemoveUntil(MaterialPageRoute(builder: (BuildContext context) => LoginPage()), (Route<dynamic> route) => false);
},
child: Text("Log Out", style: TextStyle(color: Colors.white)),
),
],
),
body: Center(child: Text("Main Page")),
drawer: Drawer(),
);
}
}```

Answer 1

change the line like this for the compiler to know that you will initialize it later:
late SharedPreferences sharedPreferences;

Answer 2

This should do the trick, if you don't intend to initialize sharedPref right away.
SharedPreferences? sharedPreferences;
Dart sound null safety has been enabled as default in flutter since 2.2 if I'm correct.
Check out this article for a starter on the topic.
https://dart.dev/null-safety

Answer 3

From the Dart documentation: "The late modifier lets you use non-nullable types and final in places you otherwise might not be able to, at the expense of runtime checking. It also gives you lazy-initialized fields." https://dart.dev/null-safety/understanding-null-safety
Your SharedPreferences declaration should have the late modifier:
late SharedPreferences sharedPreferences;