'Invalid constant value error' of Flutter

Although a variable is initialized in initState, I encountered the ‘Invalid constant value’ problem.

Could you please explain why this occurs?

I’ve annotated key points in the code.

Thank you.

P.S: I’m using Flutter 3.16.5 and Dart 3.2.3

class _LoadingScreenState extends State<LoadingScreen>
    with TickerProviderStateMixin {
  
  AnimationController? _animationController; // !Declare!

  @override
  void initState() {
    _animationController = AnimationController(   // !Initialization in initState!
      vsync: this,          
    );
  }

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

  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: ClipRect(
        child: Stack(children: [
          WelcomeView(
            animationController: _animationController!,  // !I got invalid constant value issue here! 
          ),
        ]),
      ),
    );
  }
}

>Solution :

Remove the const keyword before the Scaffold widget.

@override
  Widget build(BuildContext context) {
    return Scaffold(
       
  }