Custom Menus: handling items who's CPT doesn't exist anymore

[scribu]

Steps to reproduce:

1. Register a custom post type
2. Create a post with that type
3. Add that post to a custom menu
4. Unregister the custom post type

You will get a notice:

Notice: Trying to get property of non-object in /home/cristi/svn/wp/wp-includes/nav-menu.php on line 565

which is here:

$object = get_post_type_object( $menu_item->object );

$menu_item->type_label = $object->labels->singular_name;

The $object variable will be null in this case.

There must be a duplicate ticket of this.. I cant find it as usual, but I've seen it reported many times..

#13958