getting an error : NoSuchMethodError: invalid member on null: 'length' in flutter web

I am using AnimatedSize fade to switch between two screens. When I switch from screen 2 to screen 1, I get the error. Error image

  Widget build(BuildContext context) {
return Container(
  child: ScopedModelDescendant<Screen>(builder: (_, child, model) {
    return AnimatedSizeAndFade(
      vsync: this,
      child: model.showScreen ? AddBrand() : DemoTableLatestNew(),
      fadeDuration: const Duration(milliseconds: 300),
      sizeDuration: const Duration(milliseconds: 600),
    );
 
  }),
);

}