Default GroupButtonController must be created in _GroupButtonBodyState

[slavap]

The following code is wrong, because it leads to default GroupButtonController rebuild on every GroupButton.build(), so I have to always pass external GroupButtonController to GroupButton constructor.

return GroupButtonBody(
  ...
  controller: controller ?? GroupButtonController(),
);

The right code should be like this:

class _GroupButtonBodyState extends State<GroupButtonBody> {
  GroupButtonController _controller = widget.controller ?? GroupButtonController();

  @override
  void didUpdateWidget(covariant GroupButtonBody oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.controller != widget.controller) _controller = widget.controller ?? GroupButtonController();  
  }