Cleanup some cache entries

src/apply_mutations.rs

@@ -91,6 +91,8 @@ pub fn apply_mutations(
                 let new = stack.drain((stack.len() - m)..).collect::<Vec<Entity>>();
                 existing_parent.insert_children(existing_index, &new);
                 DespawnRecursive { entity: existing }.apply(world);
+                let existing_element_id = bevy_ui_entity_to_element_id.remove(&existing).unwrap();
+                element_id_to_bevy_ui_entity.remove(&existing_element_id);
             }
             Mutation::InsertAfter { id, m } => todo!(),
             Mutation::InsertBefore { id, m } => todo!(),