main function Null safety

Implementation

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences prefs = await SharedPreferences.getInstance();
  bool? login = prefs.getBool("login");
  print("login:" + login.toString());
  await Firebase.initializeApp();

  SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp]);
  runApp(App(login));
}