build method Null safety
- BuildContext context,
- int index
override
    Return the child at the given index. If the child at the given index does not exist, return null.
Implementation
@override
Widget? build(BuildContext context, int index) {
  if (children.isEmpty) {
    return null;
  }
  return IndexedSemantics(index: index, child: children[index % children.length]);
}