example misses update

[derdilla]

It needs a cleaned up version of:

  @override
  void didUpdateWidget(covariant FormSwitcher oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.subForms.length != widget.subForms.length) {
      controller.dispose();
      controller = TabController(length: widget.subForms.length, vsync: this);
      widget.controller?._initialize(controller);
    }
  }