On app start

main.dart

final firstEndpointProvider = Provider<String>((_) {
  throw Exception('Provider "firstEndpointProvider" was not initialized.');
});

final secondEndpointProvider = Provider<String>((_) {
  throw Exception('Provider "secondEndpointProvider" was not initialized.');
});

final sharedPrefsProvider = Provider<SharedPreferences>((_) {
  throw Exception('Provider "sharedPrefsProvider" was not initialized.');
});

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  runApp(
    const MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.yellow,
        body: Center(
          child: CircularProgressIndicator(color: Colors.red),
        ),
      ),
    ),
  );

  final data1 = await Future.delayed(const Duration(milliseconds: 1500), () => 'first');
  final data2 = await Future.delayed(const Duration(milliseconds: 1500), () => 'second');
  final prefs = await SharedPreferences.getInstance();

  runApp(
    ProviderScope(
      overrides: [
        firstEndpointProvider.overrideWithValue(data1),
        secondEndpointProvider.overrideWithValue(data2),
        sharedPrefsProvider.overrideWithValue(prefs),
      ],
      child: const App(),
    ),
  );
}