bevy_ecs: flush entities after running observers and hooks in despawn

Author: jrobsonchase

crates/bevy_ecs/src/world/entity_ref.rs

@@ -1319,6 +1319,10 @@ impl<'w> EntityWorldMut<'w> {
             }
         }
 
+        // Observers and on_remove hooks may reserve new entities, which
+        // requires a flush before Entities::free may be called.
+        world.flush_entities();
+
         for component_id in archetype.components() {
             world.removed_components.send(component_id, self.entity);
         }