childCount property Null safety
The maximum number of children that can be provided to RenderListWheelViewport.
If non-null, the children will have index in the range
[0, childCount - 1].
If null, then there's no explicit limits to the range of the children except that it has to be contiguous. If childExistsAt for a certain index returns false, that index is already past the limit.
@override int? get childCount => (widget as ListWheelViewport).childDelegate.estimatedChildCount;