Menus Administration screen, notice error when adding an item

[imath]

When not using the customizer to set a nav menu, adding a new menu item is generating this notice error :
Notice: Undefined index: menu in pathto\wp-admin\includes\ajax-actions.php on line 1137

That's because in /wp-admin/js/nav-menu.js the function addItemToMenu tries to get the #menu ID that is not existing if there is less than one menu to edit. In this case the select box to select a menu to edit is not displayed.

In other words, if you have this select box (more than 1 nav menu) :

There is no notice error.

But if you have no nav menu or only one, this select box doesn't appear and you only have this :

In this case you get the notice error

Before 4.3 dev cycle, there was a #menu ID in an hidden field of the form#update-nav-menu. I guess it has been removed due to duplicate ids as soon as the select box appears.

Using the attached patch seems to fix the issue.

[ocean90]

Related: #31816, [32695].

[metodiew]

I still have the notice when I applied the patch. Wouldn't be easier if we update $_POST['menu'] with $_POST['menu-item']? This patch fixed the notice for me.

[obenland]

In 33161:

Nav menus: Restore menu id in hidden input.

wp-admin/js/nav-menu.js uses the value as context for quick searches and when
adding menu items.

Reverts [32695], see #31816.
Fixes #32699.

[ocean90]

The label "Select a menu to edit:" is now linked with a hidden input field. We should add a new ID to the select form and use the same ID in the for attribute.

[ocean90]

In 33174:

Nav menus: Add an ID attribute to the menu select box to link it with the label.

see #31816.
fixes #32699.