# B0003

As commands are executed asynchronously, it is possible to issue a command on an entity that will no longer exist at the time of the command execution.

Erroneous code example:

```rust,should_panic
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, despawning)
        .add_systems(Update, use_entity.after(despawning))
        .run();
}

#[derive(Resource)]
struct MyEntity(Entity);

#[derive(Component)]
struct Hello;

fn setup(mut commands: Commands) {
    let entity = commands.spawn_empty().id();
    commands.insert_resource(MyEntity(entity));
}

fn despawning(mut commands: Commands, entity: Option<Res<MyEntity>>) {
    if let Some(my_entity) = entity {
        commands.entity(my_entity.0).despawn();
        commands.remove_resource::<MyEntity>();
    }
}

fn use_entity(mut commands: Commands, entity: Option<Res<MyEntity>>) {
    if let Some(my_entity) = entity {
        commands.entity(my_entity.0).insert(Hello);
    }
}
```

This will panic, as system `use_entity` is executed after system `despawning`. Without the system ordering specified here, the ordering would be random and this code would panic half the time.

The default panic message is telling you the entity id (`1v0`) and the command that failed (adding a component `Hello`):

```text
thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
```

But you don't know which system tried to add a component, and which system despawned the entity.

To get the system that created the command that panics, you can enable the `trace` feature of Bevy. This will add a panic handler that will print more information:

```text
   0: bevy_ecs::system::commands::system_commands
           with name="use_entity_after_despawn::use_entity"
             at crates/bevy_ecs/src/system/commands/mod.rs:117
   1: bevy_ecs::world::schedule
           with name=Update
             at crates/bevy_ecs/src/world/mod.rs:1755
   2: bevy_ecs::schedule::executor::single_threaded::system
           with name="bevy_app::main_schedule::Main::run_main"
             at crates/bevy_ecs/src/schedule/executor/single_threaded.rs:98
   3: bevy_ecs::world::schedule
           with name=Main
             at crates/bevy_ecs/src/world/mod.rs:1755
   4: bevy_app::app::main app
             at crates/bevy_app/src/app.rs:249
   5: bevy_app::app::update
             at crates/bevy_app/src/app.rs:246
   6: bevy_winit::winit event_handler
             at crates/bevy_winit/src/lib.rs:338
   7: bevy_app::app::bevy_app
             at crates/bevy_app/src/app.rs:290
thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
```

From the first two lines, you now know that it panics while executing a command from the system `use_entity`.

To get the system that created the despawn command, you can enable DEBUG logs for crate `bevy_ecs`, for example by setting the environment variable `RUST_LOG=bevy_ecs=debug`. This will log:

```text
DEBUG schedule{name=Main}:system{name="bevy_app::main_schedule::Main::run_main"}:schedule{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world::entity_ref: Despawning entity 1v0
thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
```

From the first line, you know the entity `1v0` was despawned when executing a command from system `despawning`. In a real case, you could have many log lines, you will need to search for the exact entity from the panic message.

Combining those two, you should get enough information to understand why this panic is happening and how to fix it:

```text
DEBUG schedule{name=Main}:system{name="bevy_app::main_schedule::Main::run_main"}:schedule{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world::entity_ref: Despawning entity 1v0
   0: bevy_ecs::system::commands::system_commands
           with name="use_entity_after_despawn::use_entity"
             at crates/bevy_ecs/src/system/commands/mod.rs:117
   1: bevy_ecs::world::schedule
           with name=Update
             at crates/bevy_ecs/src/world/mod.rs:1755
   2: bevy_ecs::schedule::executor::single_threaded::system
           with name="bevy_app::main_schedule::Main::run_main"
             at crates/bevy_ecs/src/schedule/executor/single_threaded.rs:98
   3: bevy_ecs::world::schedule
           with name=Main
             at crates/bevy_ecs/src/world/mod.rs:1755
thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
```
