Fix custom relations panics with parent/child relations (#19341)

# Objective

Fixes #18905

## Solution

`world.commands().entity(target_entity).queue(command)` calls
`commands.with_entity` without an error handler, instead queue on
`Commands` with an error handler

## Testing

Added unit test

Co-authored-by: Heart <>

Author: HeartofPhos

crates/bevy_ecs/src/relationship/mod.rs

@@ -158,19 +158,21 @@ pub trait Relationship: Component + Sized {
             {
                 relationship_target.collection_mut_risky().remove(entity);
                 if relationship_target.len() == 0 {
-                    if let Ok(mut entity) = world.commands().get_entity(target_entity) {
+                    let command = |mut entity: EntityWorldMut| {
                         // this "remove" operation must check emptiness because in the event that an identical
                         // relationship is inserted on top, this despawn would result in the removal of that identical
                         // relationship ... not what we want!
-                        entity.queue(|mut entity: EntityWorldMut| {
-                            if entity
-                                .get::<Self::RelationshipTarget>()
-                                .is_some_and(RelationshipTarget::is_empty)
-                            {
-                                entity.remove::<Self::RelationshipTarget>();
-                            }
-                        });
-                    }
+                        if entity
+                            .get::<Self::RelationshipTarget>()
+                            .is_some_and(RelationshipTarget::is_empty)
+                        {
+                            entity.remove::<Self::RelationshipTarget>();
+                        }
+                    };
+
+                    world
+                        .commands()
+                        .queue(command.with_entity(target_entity).handle_error_with(ignore));
                 }
             }
         }
@@ -424,4 +426,63 @@ mod tests {
 
         // No assert necessary, looking to make sure compilation works with the macros
     }
+
+    #[test]
+    fn parent_child_relationship_with_custom_relationship() {
+        use crate::prelude::ChildOf;
+
+        #[derive(Component)]
+        #[relationship(relationship_target = RelTarget)]
+        struct Rel(Entity);
+
+        #[derive(Component)]
+        #[relationship_target(relationship = Rel)]
+        struct RelTarget(Entity);
+
+        let mut world = World::new();
+
+        // Rel on Parent
+        // Despawn Parent
+        let mut commands = world.commands();
+        let child = commands.spawn_empty().id();
+        let parent = commands.spawn(Rel(child)).add_child(child).id();
+        commands.entity(parent).despawn();
+        world.flush();
+
+        assert!(world.get_entity(child).is_err());
+        assert!(world.get_entity(parent).is_err());
+
+        // Rel on Parent
+        // Despawn Child
+        let mut commands = world.commands();
+        let child = commands.spawn_empty().id();
+        let parent = commands.spawn(Rel(child)).add_child(child).id();
+        commands.entity(child).despawn();
+        world.flush();
+
+        assert!(world.get_entity(child).is_err());
+        assert!(!world.entity(parent).contains::<Rel>());
+
+        // Rel on Child
+        // Despawn Parent
+        let mut commands = world.commands();
+        let parent = commands.spawn_empty().id();
+        let child = commands.spawn((ChildOf(parent), Rel(parent))).id();
+        commands.entity(parent).despawn();
+        world.flush();
+
+        assert!(world.get_entity(child).is_err());
+        assert!(world.get_entity(parent).is_err());
+
+        // Rel on Child
+        // Despawn Child
+        let mut commands = world.commands();
+        let parent = commands.spawn_empty().id();
+        let child = commands.spawn((ChildOf(parent), Rel(parent))).id();
+        commands.entity(child).despawn();
+        world.flush();
+
+        assert!(world.get_entity(child).is_err());
+        assert!(!world.entity(parent).contains::<RelTarget>());
+    }
 }