Removing `Observer` panics the next time its event is triggered.

[gwafotapa]

Bevy version

0.16.1

The issue

Removing the Observer component without despawning the entity panics the next time its event is triggered.

use bevy::prelude::*;

#[test]
fn remove_observer_component() {
    #[derive(Event)]
    struct EventA;

    let mut world = World::new();
    let observer = Observer::new(|_: Trigger<EventA>| {});
    let mut entity_commands = world.spawn(observer);
    entity_commands.remove::<Observer>();
    world.trigger(EventA);
}

fn main() {}

If I'm correct, when an Observer is spawned, an ObserverState component is added to the entity as well. The world only unregisters an observer on the removing of ObserverState which does not happen in this case.

How can it be fixed?

I got around the issue by spawning the observer as a separate entity so I could despawn it and keep my original entity around. Maybe it could be fixed by adding an on_remove() hook to Observer but I'm not sure it's desirable as this is a rare scenario that you can get around easily. If it stays how it is however, it should be documented because it's natural when you realise that an observer is an entity like any other to insert or remove components.