NullableIndexedWidgetBuilder typedef Null safety
Signature for a function that creates a widget for a given index, e.g., in a list, but may return null.
Used by SliverChildBuilderDelegate.builder and other APIs that use lazily-generated widgets where the child count is not known ahead of time.
Unlike most builders, this callback can return null, indicating the index
is out of range. Whether and when this is valid depends on the semantics
of the builder. For example, SliverChildBuilderDelegate.builder returns
null when the index is out of range, where the range is defined by the
SliverChildBuilderDelegate.childCount; so in that case the
parameter's value may determine whether returning null is valid or not.
typedef NullableIndexedWidgetBuilder = Widget? Function(BuildContext context, int index);