build method Null safety
Returns the child with the given index.
Should return null if asked to build a widget with a greater index than exists. If this returns null, estimatedChildCount must subsequently return a precise non-null value (which is then used to implement RenderSliverBoxChildManager.childCount).
The values returned by this method are cached. To indicate that the widgets have changed, a new delegate must be provided, and the new delegate's shouldRebuild method must return true.
Widget? build(BuildContext context, int index);