所以我尝试了共享首选项让用户在他们注销之前仍然登录但是当我尝试重新启动应用程序时会出现这个

[VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
#0      MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:142:86)
#1      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:148:36)
#2      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:331:12)
#3      MethodChannel.invokeMapMethod (package:flutter/src/services/platform_channel.dart:358:49)
#4      MethodChannelSharedPreferencesStore.getAll (package:shared_preferences_platform_interface/method_channel_shared_preferences.dart:44:25)
#5      SharedPreferences._getSharedPreferencesMap (package:shared_preferences/shared_preferences.dart:180:57)
#6      SharedPreferences.getInstance (package:shared_preferences/shared_preferences.dart:56:19)
#7      read_loggedIn (package:indonesia_bisa_2/API/LocalService.dart:11:41)
#8      main (package:indonesia_bisa_2/Main/main.dart:16:14)
#9      _runMainZoned.<anonymous closure>.<…>

这是检查器代码

import 'package:shared_preferences/shared_preferences.dart';

Future<void> save_loggedIn() async {
  final prefs = await SharedPreferences.getInstance();
  prefs.setString("loggedIn", "true");
}

//this is for checking the saved variable to check if the user is already logged in or not
Future<String> read_loggedIn() async {
  final prefs = await SharedPreferences.getInstance();
  var value = prefs.getString("loggedIn");
  return value;
}

这是使用代码的时间

Future<void> main() async {
  runApp(new MaterialApp(
      debugShowCheckedModeBanner: false,
      home: (read_loggedIn() == "true") ? new SPAL (): new MyApp()));
}

回答

void main() async {
  await runZonedGuarded(
    () async {
      WidgetsFlutterBinding.ensureInitialized();

      final prefs = await SharedPreferences.getInstance();
      runApp(MyApp(prefs));
    },
    (error, st) => print(error),
  );
}