Why do I get grey screen when hosting Flutter app?

I've checked all the other answers about grey screen when hosting a web app but I have not found a solution for my problem yet. I have checked my code for errors, and fixed the ones that I had.

When I deploy my app, everything seems fine. The loginscreen appears and I can succesfully log in and get directed to my homescreen. But when I refresh my browser the screen turns grey and I have to deploy my app again to get it to work.

Hope somebody has some clue to what this can be?

Some further testing in Debugmode get me this error:

The following TypeErrorImpl was thrown building Builder: Unexpected null value. The relevant error-causing widget was: MaterialApp MaterialApp:file:///C:/Users//lib/main.dart:50:12

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences sharedpreference  = await SharedPreferences.getInstance();
  sharedpreference.getString('email');
  await Firebase.initializeApp( options: const FirebaseOptions(
    apiKey: "AI******YSpsnJ8",
    appId: "1:981*******50",
    messagingSenderId: "******",
    projectId: "tdfsfdf**",
  ),
  );
  if (kIsWeb) {
    print('Web');
    SystemChrome.setPreferredOrientations(
        [DeviceOrientation.portraitUp, DeviceOrientation.portraitDown]
    ).then((_) => runApp(const MyApp()));
  } else {
    print('mobil');
    // NOT running on the web! You can check for additional platforms here.
    runApp(const MyApp());
}
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);


  @override
  Widget build(BuildContext context) {
    return MaterialApp(

      title: 'Flutter Demo',
     home: FirebaseAuth.instance.currentUser == null
          ? const UserLoginPage()
          : const UserHomePage(),
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),

      initialRoute: UserLoginPage.id,
      routes:{
        UserLoginPage.id : (context) => const UserLoginPage(),
        AdminServicePage.id : (context) => const AdminServicePage(),
        AdminToolsPage.id : (context) => const AdminToolsPage(),
        AdminDeviationPage.id : (context) => const AdminDeviationPage(),
        AdminUsersPage.id : (context) => const AdminUsersPage(),
        UserHomePage.id : (context) => const UserHomePage(),
        UserToolListPage.id : (context) => const UserToolListPage(),
        WebHomePage.id : (context) => const WebHomePage(),
        WebOverviewPage.id : (context) => const WebOverviewPage(),
       
      },
      debugShowCheckedModeBanner: false,

    );
  }
}