Intermittent "entity does not exist" panic

[sephelps]

My implementation of a loading screen is experiencing an intermittent panic:

thread 'Compute Task Pool (14)' panicked at 'Attempting to create an EntityCommands for entity 2v0, which doesn't exist.', /home/sephelps/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_transform-0.6.0/src/hierarchy/hierarchy_maintenance_system.rs:69:18

1. Conceptually, when entering the Loading state, I spawn all the entities for the loading screen--like the progress bar.
2. On update during the Loading state, I check to see if the assets are loaded, and if so, set the state to the next state.
3. On exit of the Loading state, I despawn the entities created when entering the Loading state.

Not every time, but sometimes, when attempting to despawn the loading screen entities, it claims that they don't exist, even though I'm getting the entity ID from a Query in the same function.

When the panic occurs, I also get this warning:

2022-04-11T03:27:29.890556Z WARN bevy_ui::flex: Styled child in a non-UI entity hierarchy. You are using an entity with UI components as a child of an entity without UI components, results may be unexpected.

I'm assuming it has to do with parallel system running, but I thought the on_enter, on_exit, etc. were hard boundaries. Even when specifying labels and after, I still receive the panic sometimes. Loading a sufficiently large amount of assets stops the error.

Here is the structure of the app's systems:

impl Plugin for LoadingPlugin {
    fn build(&self, app: &mut App) {
        let spawn = SystemSet::on_enter(AppState::Loading)
            .with_system(spawn_loading);

        let update = SystemSet::on_update(AppState::Loading)
            .with_system(update_loading_progress)
            .with_system(check_loading_done);

        let unload = SystemSet::on_exit(AppState::Loading)
            .with_system(unload_loading);

        app.add_system_set(spawn)
            .add_system_set(update)
            .add_system_set(unload);
    }
}

Here is the entity spawning system:

fn spawn_loading(
    mut commands: Commands,
) {
    let root = commands.spawn_bundle(NodeBundle {
        style: Style {
            size: Size::new(Val::Percent(100.), Val::Percent(100.)),
            align_items: AlignItems::Center,
            justify_content: JustifyContent::Center,
            ..Default::default()
        },
        color: Color::NONE.into(),
        ..Default::default()
    }).insert(UiRoot).id();

    let container = commands.spawn_bundle(NodeBundle {
        style: Style {
            size: Size::new(Val::Px(200.), Val::Px(15.)),
            ..Default::default()
        },
        color: Color::BLACK.into(),
        ..Default::default()
    }).insert(Parent(root)).id();

    commands.spawn_bundle(NodeBundle {
        style: Style {
            size: Size::new(Val::Percent(0.), Val::Percent(100.)),
            ..Default::default()
        },
        ..Default::default()
    }).insert(UiProgress).insert(Parent(container));
}

Here is the function that triggers the on_exit SystemSet:

fn check_loading_done(
    loading: Res<LoadingAssets>,
    server: Res<AssetServer>,
    mut state: ResMut<State<AppState>>,
    next_state: Res<NextState>,
) {
    if server.get_group_load_state(loading.0.iter().map(|h| h.id)) == LoadState::Loaded {
        state.set(next_state.0);
    }
}

The system where this intermittent panic occurs:

fn unload_loading(
    mut commands: Commands,
    mut ui_query: Query<Entity, With<UiRoot>>,
) {
    // how is it possible `ui_query.single()` returns an entity but `commands.entity()` thinks it doesn't exist (and only sometimes)?
    commands.entity(ui_query.single()).despawn_recursive();
    // ...
}

[DJMcNab]

This seems likely to be a duplicate of #3195

[sephelps]

Unfortunately it happens when I despawn a single entity as well, but I have figured out a fix. Thanks!

[sephelps]

I was able to fix this by making the spawning and despawning systems exclusive:

impl Plugin for LoadingPlugin {
    fn build(&self, app: &mut App) {
        let spawn = SystemSet::on_enter(AppState::Loading)
            .with_system(spawn_loading.exclusive_system());

        let update = SystemSet::on_update(AppState::Loading)
            .with_system(update_loading_progress)
            .with_system(check_loading_done);

        let unload = SystemSet::on_exit(AppState::Loading)
            .with_system(unload_loading.exclusive_system());

        app.add_system_set(spawn)
            .add_system_set(update)
            .add_system_set(unload);
    }
}