parentOf method
Returns the parent cluster if element
has one.
Implementation
@override
ImmutableLayerCluster<T>? parentOf(LayerElement<T> element) {
element as ImmutableLayerElement<T>;
if (element.parentId == -1) return null;
final parentZoom = getOriginZoom(element.parentId) - 1;
return _trees[parentZoom]!.parentOf(element);
}