build method
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 (childCount == null) {
final Widget child = builder(context, index);
return IndexedSemantics(child: child, index: index);
}
if (index < 0 || index >= childCount!) return null;
return IndexedSemantics(child: builder(context, index), index: index);
}