The Scaffold implementation uses a function to return a Widget here which is both:

• Not needed, because the function is used only once (in the build method) and nowhere else
• Using functions to return widgets is a bad practice as stated by the official docs here, here and here.

This

child: _wrapBottomSheet(
  BottomSheet(
    ...
  ),
),

should become

child: Semantics(
  container: true,
  onDismiss: close,
  child:  NotificationListener<DraggableScrollableNotification>(
    onNotification: extentChanged,
    child: BottomSheet(
      ...
    ),
  ),
);