I found this bug after turned on WP_DEBUG.
If we add Register menu item into navigation menu, and login, WordPress will throw this bug.
It is because the statement at line 40 refer to $item object
$item->url = site_url( 'wp-login.php?action=register', 'login' );
which has been set to null before.
Beside, return null from filter of wp_setup_nav_menu_item is illegal.
Thank you for creating this plugin. It is great.