wp_setup_nav_menu_item() throws PHP warning when using virtual menu-items

[apedog]

I believe this bug was introduced in 47211

TLDR

wp_setup_nav_menu_item() calls get_post() but does no sanity checking before calling get_post_states() with what might be a null. Adding a simple if ( $menu_post !== null ) before calling get_post_states() resolves this issue.

I've added a patch.

Details

I have an old plugin (pre-dating above commit) that renders custom dynamically-generated drop-down menus (eg. recent posts, yearly archives).
The plugin is somewhat based on ​https://www.ibenic.com/understanding-wordpress-menu-items/ but is a bit more involved. It's a stable and simple plugin.

The way the plugin works is it adds a single menu-item meta-box in the Menu editor page. However on the front it dynamically generates submenu items that don't show on the Edit Menu page. On the front the plugin creates virtual/fake WP_Post objects at runtime that are fed into Walker_Nav_Menu_Checklist. Therefore these objects have no corresponding $menu_post in the database.

Since WordPress 5.4, core throws multiple Attempt to read property "post_status|ID" on null notices on every page load. These notices have been upgraded to warnings in PHP 8.

wp-admin/includes/template.php:2288
get_post_states()
wp-includes/nav-menu.php:839
wp_setup_nav_menu_item()
wp-includes/nav-menu.php:839
array_map()
wp-includes/nav-menu.php:727
wp_get_nav_menu_items()
wp-admin/nav-menus.php:596

[apedog]

Perhaps a similar sanity check could be added to get_post_states() if parameter $post is null.