How to properly declare method in custom Flutter class - flutter

Question

I have created a custom class from the Floating Action Bubble package, to make it reusable. Now, if I declare the method customBubble as shown below, (which I am not sure it is the right way), everything works fine, apart the fact that I cannot access _animationController!.reverse() before the initState.
Else, if I declare the method customBubble inside class _FabAddCategoryState, the problem is that I am not able to access it from other class/screens.
Can you help me to write it in the right way?
the code:
class FabAddCategory extends StatefulWidget {
final List<Bubble>? items ;
const FabAddCategory({Key? key, this.items}) : super(key: key);
#override
State<FabAddCategory> createState() => _FabAddCategoryState();
Bubble customBubble({required IconData icon, TextStyle? titleStyle, Color? iconColor,
required String title, MaterialColor? bubbleColor, required Null Function() onPress, }) {
return Bubble(icon: icon, iconColor: Colors.white, title: title,
titleStyle: const TextStyle(fontSize: 16, color: Colors.white),
bubbleColor: Colors.amber,
onPress: (){
//_animationController!.reverse();
onPress.call();
});
}
}
class _FabAddCategoryState extends State<FabAddCategory> with SingleTickerProviderStateMixin{
Animation<double>? _animation;
AnimationController? _animationController;
#override
void initState() {
_animationController = AnimationController(
vsync: this,
duration: const Duration(milliseconds: 260),
);
final curvedAnimation = CurvedAnimation(curve: Curves.easeInOut, parent: _animationController!);
_animation = Tween<double>(begin: 0, end: 1).animate(curvedAnimation);
super.initState();
}
#override
Widget build(BuildContext context) {
return FloatingActionBubble(
items: widget.items!,
animation: _animation!,
onPress: () =>
_animationController!.isCompleted ?
_animationController!.reverse()
: _animationController!.forward(),
iconColor: Colors.white, backGroundColor: Theme.of(context).primaryColor,
iconData: Icons.menu,
);
}
}