Adding an entity to a level, and re-checkpointing, in load-checkpoint crashes dustforce

[JonatanWaern]

class script {
    scene@ g;

    script() {
        @g = get_scene();
    }

    void checkpoint_load() {
        entity@ new_apple = create_entity("hittable_apple");
        new_apple.x(0);
        new_apple.y(0);
        g.add_entity(new_apple);
        g.save_checkpoint(0, 0, true);
    }
}

Will crash when loading a checkpoint