How do I get a parent menu in the WordPress Admin panel which has no action?

The ‘null’ as the callback is supposed to make the top level menu item do nothing.

You can’t! That link will always lead somewhere and that cannot be avoided ( and would make no sense and have accessibility issues if you somehow achieved it anyway ).

Instead create a menu page and a submenu page that have the same identifier. This way when you click on the main link the user finds themself on the first submenu page. This is how most people do it, how the menu is designed, and how post types and settings already work.

e.g. it might look something like this:

$menu_slug = 'you-menu-slug';
add_menu_page(
    "Results",
    'WP Docs Orders',
    'edit_published_posts',
    $menu_slug,
    false
);
add_submenu_page(
    $menu_slug,
    "Classes - Results",
    "Classes - Results",
    'edit_published_posts',
    $menu_slug,
    [ $this, 'load_class_results' ]
);

I would also note, that the use of null may actually trigger PHP deprecation warnings in some newer versions of PHP, and even fatal errors in newer ones still.

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