Observers and hooks doesn't pair well with mapped entities

[Shatur]

Bevy version

v0.14.0

What you did

I have a component that stores an entity. The component implements and reflects MapEntities, so on scene deserialization it automatically maps entities inside it.

What went wrong

When I implement on_add hook or OnAdd observer for such component, it triggers before MapEntities which results in invalid entity access.
It happens because we insert components here:

bevy/crates/bevy_scene/src/scene.rs

Lines 117 to 123 in eabb58a

	reflect_component.copy(
	&self.world,
	world,
	scene_entity.id(),
	*entity,
	&type_registry,
	);

But entities mapped later:

bevy/crates/bevy_scene/src/scene.rs

Lines 128 to 131 in eabb58a

	for registration in type_registry.iter() {
	if let Some(map_entities_reflect) = registration.data::<ReflectMapEntities>() {
	map_entities_reflect.map_all_entities(world, entity_map);
	}

I think we should map entities before the insertion.

[Shatur]

I would like to add a comment by @mockersf from Discord:

that's going to be not fun to fix, we need to add the entities before we can map them... so I guess the fix would be to spawn empty entities, do the mapping, then inserting the components...