Flutter - 如何处理小部件属性的非空实例

Question

I am new to flutter and currently trying to simple animation on my component, but i keep getting a " Non-Nullable Instance " error, i checked out other video tutorials but theirs worked perfectly while mine keeps throwing errors.

Here's my code.

class HomeScreen extends StatefulWidget {
  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> with TickerProviderStateMixin {

  Animation<Offset> sbarAnimation;
  AnimationController sbarAnimationController;

  @override

  void initState() {

    super.initState();

    sbarAnimationController = AnimationController(
      vsync: this,
      duration: Duration(milliseconds: 250),
    );

    sbarAnimation = Tween<Offset>(
      begin: Offset(-1, 0),
      end: Offset(0, 0),
    ).animate(
      CurvedAnimation(
        parent: sbarAnimationController,
        curve: Curves.easeInOut,
      ),
    );

  }

And then the code for the widget is below

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

      body: Container(
        color: kBackgroundColor,
        child: Stack(

          children: [
            SafeArea(
              //so the background covers the entire app..
              child: Column(
                children: [
                  HomeScreenNavBar(),                     
                  ExploreCourseList(),
                ],
              ),
            ),

            SlideTransition(

              position: sbarAnimation,   // this is where i call the _animation

              child: SafeArea(
                child: SidebarScreen(),
                bottom: false,  
              ),
            ),
          ],

        ),
      ),
    );
  }
}

below is the errors i keep getting, and most of the solutions i've seen did exactly the same thing, theirs works, but mine is giving this errors.. please what am i supposed to do.

Answer 1

There two way to solve the issue.

1. Which already answered by Juan Carlos Ramón Condezo , using late keyword, where you must have to initialize it before using anywhere.

2. You can make the variable nullable. By using '?' After declaring the variable type.

Animation?<Offset> sbarAnimation;
AnimationController? sbarAnimationController;

So, you can check if the variable is null and initialize it when needed.

Answer 2

Use late keyword to initialize variables later as you are doing in your initState

late Animation<Offset> sbarAnimation;
late AnimationController sbarAnimationController;