Null check operator used on a null value

[ManasMalla]

The bug: Null check operator used on a null value
The following _CastError was thrown while dispatching notifications for ThemeProvider: Null check operator used on a null value

When the exception was thrown, this was the stack
#0      Element.widget
package:flutter/…/widgets/framework.dart:3203
#1      Provider._inheritedElementOf
package:provider/src/provider.dart:341
#2      Provider.of
package:provider/src/provider.dart:293
#3      AppTheme.init.<anonymous closure>
package:******/providers/theme_provider.dart:99
#4      ChangeNotifier.notifyListeners
package:flutter/…/foundation/change_notifier.dart:308
#5      ThemeProvider.setThemeMode
package:******/providers/theme_provider.dart:15
<asynchronous suspension>
The ThemeProvider sending notification was: Instance of 'ThemeProvider'

To Reproduce

theme_provider.dart

class ThemeProvider extends ChangeNotifier {
  bool isDarkThemeEnabled = false;
  setThemeMode(ThemeMode? themeMode) async {
    SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
    if (themeMode == ThemeMode.dark) {
      isDarkThemeEnabled = true;
    } else {
      isDarkThemeEnabled = false;
    }
    await sharedPreferences.setBool("theme_mode", isDarkThemeEnabled);
    notifyListeners();
    print("Changing data $isDarkThemeEnabled");
  }
}

class AppTheme {
.....
  init(context) {
    Provider.of<ThemeProvider>(context, listen: true).addListener(() {
      themeMode =
          Provider.of<ThemeProvider>(context, listen: false).isDarkThemeEnabled;
      print("ThemeMode2: $themeMode");
....
});
}

main.dart

Future<void> setPrefThemeMode(BuildContext context) async {
  SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
  var system_themeMode =
      WidgetsBinding.instance?.window.platformBrightness == Brightness.dark;
  var pref_themeMode =
      (sharedPreferences.getBool("theme_mode") ?? system_themeMode);
  var themeMode = system_themeMode ? ThemeMode.dark : ThemeMode.light;
  print("systeMode:$system_themeMode");
  if (system_themeMode == false && pref_themeMode == true) {
    //themeMode = ThemeMode.dark;
  }
  Provider.of<ThemeProvider>(context, listen: false).setThemeMode(themeMode);
}


class MyApp extends StatefulWidget {
  //const MyApp({Key? key}) : super(key: key);
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override
  void initState() {
    // connect();
    super.initState();
    WidgetsBinding.instance?.addObserver(this);
    WidgetsBinding.instance?.addPostFrameCallback((_) {
      setPrefThemeMode(context);
    });
  }

  @override
  void dispose() {
    WidgetsBinding.instance?.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangePlatformBrightness() {
    super.didChangePlatformBrightness();
    print("ThemeModeChanged");
    setPrefThemeMode(context);
  }

  @override
  Widget build(BuildContext context) {
    AppTheme().init(context);
    return MaterialApp(debugShowCheckedModeBanner: false, home: SplashScreen());
  }
}

Expected behavior
The isDarkThemeEnabled boolean value must return the correct value and be applied to the var themeMode in AppTheme class for further changes.

[rrousselGit]

It's a Flutter error, not a provider one.

This happens when you're using the context of a widget that is unmounted. It's likely that your widget was removed from the widget tree.

[mdmota]

OK. and how to avoid it?
If I test if(mounted) it won't guarantee my provider method is executed.

[elementlo]

Maybe using deactivate() instead will help, it is called before dispose() where you can touch the alive context to find the provider :)