_GroupButtonBodyState.initState() minor bug

[Slava P.]

class _GroupButtonBodyState extends State<GroupButtonBody> {
  @override
  void initState() {
    super.initState();

    if (!widget.isRadio) {
      widget.controller.toggleIndexes(widget.selectedButtons ?? []); // <<< has to be selectIndexes
    }

Because the following code is not working properly:

GroupButton(
  selectedButtons: selected,
  controller: GroupButtonController()..selectIndexes(selected),

It's oversimplified example, the point is that you cannot set properly synchronized selectedButtons and controller.