fix PHP notice "Trying to access array offset on null"

Fix PHP notice "Trying to access array offset on null" when doing a wp-admin request with CLI since we are in eval() and the variables are therefore not in global context

Fix https://core.trac.wordpress.org/ticket/62286

src/wp-admin/includes/menu.php

@@ -282,6 +282,8 @@ function add_menu_classes( $menu ) {
  * @param bool $custom Whether custom ordering is enabled. Default false.
  */
 if ( apply_filters( 'custom_menu_order', false ) ) {
+	// this might not be in global context if it's called from outside of global context or with eval()
+	global $menu_order, $default_menu_order;
 	$menu_order = array();
 
 	foreach ( $menu as $menu_item ) {