non-nullable variable is not initialized

Ig you need to do something like this

 static MediaQueryData _mediaQueryData = 0;
  static double screenWidth = 0;
  static double screenHeight = 0;
  static double deafualtSize = 0;
  static Orientation? orientation;
  
  void init(BuildContext context) {
    _mediaQueryData = MediaQuery.of(context);
    screenWidth = _mediaQueryData.size.width;
    screenHeight = _mediaQueryData.size.height;
    orientation = _mediaQueryData.orientation;
  }
}

Everythime you use the orientation variable you will have to call it like orientation!
This error is because of the new null safety feature. You have to either give a default value or say that the variable can be null by using a ? after the type declaration as in the example above
Hope it helps. Let me know if there is any other problem