builder property Null safety
final
Called to build children for the sliver.
Will be called only for indices greater than or equal to zero and less than childCount (if childCount is non-null).
Should return null if asked to build a widget with a greater index than exists.
May result in an infinite loop or run out of memory if childCount is null
and the builder always provides a zero-size widget (such as Container()
or SizedBox.shrink()). If possible, provide children with non-zero size,
return null from builder, or set a childCount.
The delegate wraps the children returned by this builder in RepaintBoundary widgets.
Implementation
final NullableIndexedWidgetBuilder builder;