Called to find the new index of a child based on its key in case of reordering.
If not provided, a child widget may not map to its existing RenderObject when the order of children returned from the children builder changes. This may result in state-loss.
This callback should take an input Key, and it should return the index of the child element with that associated key, or null if not found.
final ChildIndexGetter? findChildIndexCallback;