destroyed method
When an entity has been destroyed, we remove it from the list and that will automatically sync that to the observable list.
Implementation
void destroyed(Entity entity) {
final current = _entities.value;
// Drop the guid and remove the entity from the list.
final newList = Map.fromIterables([...current.keys], [...current.values].where((e) => e != entity));
// Set the new list.
_entities.add(newList);
}