Strip widgets from sidebars_widgets[] settings if not registered

A widget added to a sidebar leaves its widget_id in the sidebar even when the widget goes away, for example due to a plugin deactivation. Now any such widget remants will get filtered out when sent to the client. Without this fix, a JavaScript error would be thrown: > Uncaught TypeError: Cannot read property 'transport' of undefined

Author: westonruter

widget-customizer.php

@@ -430,7 +430,8 @@ static function customize_register( $wp_customize = null ) {
 } else {
 self::$sidebars_eligible_for_post_message[$sidebar_id] = ( 'postMessage' === self::get_sidebar_widgets_setting_transport( $sidebar_id ) );
 }
-$setting_args['sanitize_callback'] = array( __CLASS__, 'sanitize_sidebar_widgets' );
+$setting_args['sanitize_callback'] = array( __CLASS__, 'sanitize_sidebar_widgets' );
+$setting_args['sanitize_js_callback'] = array( __CLASS__, 'sanitize_sidebar_widgets_js_instance' );
 $wp_customize->add_setting( $setting_id, $setting_args );
 $new_setting_ids[] = $setting_id;
 
@@ -1314,6 +1315,19 @@ static function sanitize_widget_js_instance( $value ) {
 return $value;
 }
 
+/**
+ * Strip out widget IDs for widgets which are no longer registered, such
+ * as the case when a plugin orphans a widget in a sidebar when it is deactivated.
+ *
+ * @param array $widget_ids
+ * @return array
+ */
+static function sanitize_sidebar_widgets_js_instance( $widget_ids ) {
+global $wp_registered_widgets;
+$widget_ids = array_values( array_intersect( $widget_ids, array_keys( $wp_registered_widgets ) ) );
+return $widget_ids;
+}
+
 /**
  * Find and invoke the widget update and control callbacks. Requires that
  * $_POST be populated with the instance data.