ScrollableWidgetBuilder typedef Null safety
The signature of a method that provides a BuildContext and ScrollController for building a widget that may overflow the draggable Axis of the containing DraggableScrollableSheet.
Users should apply the
scrollController to a ScrollView subclass, such
as a SingleChildScrollView, ListView or GridView, to have the whole
sheet be draggable.
typedef ScrollableWidgetBuilder = Widget Function( BuildContext context, ScrollController scrollController, );