Observers panic when an earlier observer despawned the entity

[janhohenheim]

Bevy version

0.16.1

What you did

use bevy::prelude::*;

fn main() -> AppExit {
    App::new()
        .add_plugins(MinimalPlugins)
        .add_systems(Startup, spawn)
        .add_systems(Update, despawn_transforms)
        .add_observer(despawn)
        .add_observer(dispatch_print)
        .add_observer(print)
        .run()
}

fn spawn(mut commands: Commands) {
    commands.spawn(Transform::default());
}

fn despawn_transforms(query: Query<Entity, With<Transform>>, mut commands: Commands) {
    for entity in query.iter() {
        commands.entity(entity).trigger(Kill);
    }
}

#[derive(Event)]
struct Kill;

#[derive(Event)]
struct Print;

fn despawn(trigger: Trigger<Kill>, mut commands: Commands) {
    commands.entity(trigger.target()).despawn();
}

fn dispatch_print(trigger: Trigger<Kill>, mut commands: Commands) {
    commands.entity(trigger.target()).trigger(Print);
}

fn print(_trigger: Trigger<Print>) {
    println!("Killed something");
}

What went wrong

thread 'main' panicked at /home/hhh/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bevy_ecs-0.16.1/src/error/handler.rs:141:1:
Encountered an error in command `<bevy_ecs::system::commands::entity_command::trigger<test::Print>::{{closure}} as bevy_ecs::error::command_handling::CommandWithEntity<core::result::Result<(), bevy_ecs::world::error::EntityMutableFetchError>>>::with_entity::{{closure}}`: The entity with ID 3v1 does not exist (enable `track_location` feature for more details)

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic when applying buffers for system `test::despawn_transforms`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!

Additional information

This came up organically while working on our jam submission, where the trigger is OnDamage. If the damage is high enough, the enemy is despawned. That means that other later running observers, like the ones playing a damage sound effect, panic.
I would have expected the later observers to simply not run.

[rendaoer]

Not only that, this problem also occurs under normal despawn. I use the current bevy main.

use bevy::input::common_conditions::input_just_pressed;
use bevy::prelude::*;

fn main() -> AppExit {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, close.run_if(input_just_pressed(KeyCode::Space)))
        .run()
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2d);

    commands
        .spawn((
            Node {
                width: Val::Percent(100.0),
                height: Val::Percent(100.0),
                justify_content: JustifyContent::Center,
                align_items: AlignItems::Center,
                ..default()
            },
            Root,
            BackgroundColor(Srgba::WHITE.into()),
        ))
        .with_child((
            Node {
                width: Val::Px(100.0),
                height: Val::Px(100.0),
                justify_content: JustifyContent::Center,
                align_items: AlignItems::Center,
                ..default()
            },
            BackgroundColor(Srgba::RED.into()),
        ))
        .with_child((
            Node {
                width: Val::Px(100.0),
                height: Val::Px(100.0),
                justify_content: JustifyContent::Center,
                align_items: AlignItems::Center,
                ..default()
            },
            BackgroundColor(Srgba::BLUE.into()),
        ));
}

fn close(mut commands: Commands, root: Single<Entity, With<Root>>) {
    commands.entity(root.into_inner()).despawn();
}

#[derive(Component, Reflect, Debug)]
#[reflect(Component)]
struct Root;