Ticket #39103: 39103.0.dff

diff --git src/wp-admin/js/customize-nav-menus.js src/wp-admin/js/customize-nav-menus.js
index b7556f6..a37d2c4 100644
--- src/wp-admin/js/customize-nav-menus.js
+++ src/wp-admin/js/customize-nav-menus.js
@@ -1169,7 +1169,11 @@
 
                        // @todo It would be better if this was added directly on the setting itself, as opposed to the control.
                        control.setting.validate = function( value ) {
-                               return parseInt( value, 10 );
+                               if ( '' === value ) {
+                                       return 0;
+                               } else {
+                                       return parseInt( value, 10 );
+                               }
                        };
 
                        // Edit menu button.
diff --git src/wp-includes/class-wp-customize-nav-menus.php src/wp-includes/class-wp-customize-nav-menus.php
index ca1b72f..29c0c4d 100644
--- src/wp-includes/class-wp-customize-nav-menus.php
+++ src/wp-includes/class-wp-customize-nav-menus.php
@@ -542,10 +542,12 @@ final class WP_Customize_Nav_Menus {
                        }
                }
                $this->manager->add_dynamic_settings( $nav_menus_setting_ids );
-               foreach ( $nav_menus_setting_ids as $setting_id ) {
-                       $setting = $this->manager->get_setting( $setting_id );
-                       if ( $setting ) {
-                               $setting->preview();
+               if ( ! $this->manager->doing_ajax( 'customize_save' ) ) {
+                       foreach ( $nav_menus_setting_ids as $setting_id ) {
+                               $setting = $this->manager->get_setting( $setting_id );
+                               if ( $setting ) {
+                                       $setting->preview();
+                               }
                        }
                }