The following _CastError was thrown building Game-Null check operator used on a null value--Flutter

Question

I have a Problem with Shared Preference. How can I make a IF else operation with Shared Preference Bool Value? The Console show me: The following _CastError was thrown building Game(dirty, dependencies: [_LocalizationsScope-[GlobalKey#0453f], _InheritedTheme], state: _GameState#6a56a): Null check operator used on a null value

My Code:

    @override
  void initState() {
    super.initState();
 ...

    _sprache();
    _gesamtPkt();
  }
...

  @override
  void dispose() {
    super.dispose();
  }

  ///Loading counter value on start (load)
  _sprache() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    setState(() {
      _deutsch = (prefs.getBool('deutsch') ?? true);
      print(Text("Deutsch: $_deutsch"));
    });
  }
...

PageRouteBuilder(
                                    pageBuilder:
                                        // ignore: missing_return
                                        (context, animation1, animation2) {
                                      if (_deutsch = true) return Game();
                                      return GameEN();
                                    },

Answer 1

It doesn't look like the line that causes that error is in the code snippet, but perhaps this will help:

In Dart (with null safety enabled), there are nullable and non-nullable variables:

String nonNullable = 'hello';  // OK
String nonNullable = null;  // compile time error

String? nullable = 'hello';  // OK
String? nullable = null;  // OK

nullable.length() // compile time error, `nullable` might be null

You can't do much with a nullable variable, because if it is null, calling a method on it is an error (the classic null dereference error).

Dart is usually pretty smart about automatically converting between nullable and non-nullable types where it is safe:

String? maybeNull = ...  // some maybe null string
if (maybeNull != null) {
  // compiler can prove that maybeNull is non-null here
  print(maybeNull.length());  // this is allowed
}

However, sometimes as the programmer, you know that a variable is non-null, but the compiler cannot prove it. In these cases, you use ! to forcibly convert:

String? maybeNull = 'hello';
String notNull = maybeNull!;  // OK

String? maybeNull = null;
String uhOh = maybeNull!;  // runtime error: null-check operator used on null value

To fix your issue, go to the section of code that the error points to, and check for uses of this null check operator ! . It looks like it happened in a widget build method, so check that the value it's used on really can never be null, or handle the case where it is null (perhaps the data hasn't loaded yet?).