Flutter shared_preferences sometimes not initialized when using the Singleton pattern - one instance across all files

So I have created a class for using shared_preferences with a singleton pattern, so I can use one instance across all my files.

And almost all of the time it works flawlessly, but occasionally, when I am not using the app but it is still open in the background, and then open it, I get a gray screen because my _prefs can't get the data (probably because it isn't initialized). I can't debug test it because it only appears in release mode. I am not using any more methods from shared_preferences, like SharedPreferences.clear() or SharedPreferences.setMockInitialValues({});. Also, I am only initializing them in my main() function (code given below).

I am wondering if I am doing something wrong, not correctly initializing it or not checking if it is initialized before setting and retrieving data. Does anyone have any clue what may be wrong? Maybe I need to take a different approach?

custom class:

class SharedUserData {
  // home screen dailies
  static const String _APP_CURRENT_DATA_SEED = 'app_current_data_seed';
  static const String _CHOSEN_DAILY_PHYSICS_QUESTION_ANSWER_INDEX= 'chosen_daily_physics_question_index';

  SharedPreferences? _prefs;

  // SINGLETON
  SharedUserData._privateConstructor();
  static final SharedUserData _instance = SharedUserData._privateConstructor();
  static SharedUserData get instance => _instance;


  Future<void> init() async {
    _prefs = await SharedPreferences.getInstance();
  }

  void initializeHomeScreenDailies() {
    int? writtenDateSeed = readCurrentDataSeed();
    int calculatedDateSeed = calculateCurrentDateSeed();

    if(writtenDateSeed == null || writtenDateSeed != calculatedDateSeed) {
      writeCurrentDataSeed(calculatedDateSeed);
      writeChosenDailyPhysicsQuestionAnswerIndex(-1);
    }

    printAllData();
  }


  // READ METHODS
  int? readCurrentDataSeed() {
    assert(_prefs != null, 'SharedPreferences not initialized. Call init() first.');
    return _prefs!.getInt(_APP_CURRENT_DATA_SEED);
  }
  int? readChosenDailyPhysicsQuestionAnswerIndex() {
    assert(_prefs != null, 'SharedPreferences not initialized. Call init() first.');
    return _prefs!.getInt(_CHOSEN_DAILY_PHYSICS_QUESTION_ANSWER_INDEX);
  }

  // WRITE METHODS
  Future<void> writeCurrentDataSeed(int currentDataSeed) async {
    assert(_prefs != null, 'SharedPreferences not initialized. Call init() first.');
    await _prefs!.setInt(_APP_CURRENT_DATA_SEED, currentDataSeed);
  }
  Future<void> writeChosenDailyPhysicsQuestionAnswerIndex(int chosenDailyPhyscisQuestionAnswerIndex) async {
    assert(_prefs != null, 'SharedPreferences not initialized. Call init() first.');
    await _prefs!.setInt(_CHOSEN_DAILY_PHYSICS_QUESTION_ANSWER_INDEX, chosenDailyPhyscisQuestionAnswerIndex);
  }
}

I am initializing it in my main.dart file like so:

void main() {
  initializeSharedPreferences();

  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp());
}

void initializeSharedPreferences() async {
  WidgetsFlutterBinding.ensureInitialized();

  await SharedUserData.instance.init();
  SharedUserData.instance.initializeHomeScreenDailies();
}

And using like (in a function):

final int? seed = SharedUserData.instance.readCurrentDataSeed();

I tried researching it only but it seems like I can't find anywhere where this problem had also occured.

Since you call an async function from main, you should make main also async and use await to wait for initializeSharedPreferences to complete.

Try this:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await initializeSharedPreferences(); 
  runApp(MyApp());
}

Future<void> initializeSharedPreferences() async {
  await SharedUserData.instance.init();
  SharedUserData.instance.initializeHomeScreenDailies();
}