SizeTransition.buildTransition isn't a valid override of CustomTransition.buildTransition

Question

I got error

When I create custom transition, I got the error above. I just copy the code from here

And here is the code:

class SizeTransitions extends CustomTransition {
  @override
  Widget buildTransition(
      BuildContext context,
      Curve curve,
      Alignment alignment,
      Animation<double> animation,
      Animation<double> secondaryAnimation,
      Widget child) {
    return Align(
      alignment: Alignment.center,
      child: SizeTransition(
        sizeFactor: CurvedAnimation(
          parent: animation,
          curve: curve,
        ),
        child: child,
      ),
    );
  }
}

I not sure why it has an error.

Fix the error above. I need create an custom transition to switch page.