Widgets: Retrieve latest widgets before loading sidebars (#32997)

This fixes issues where sidebars would be unexpectedly missing from the new widgets screen. Running retrieve_widgets syncs sidebars that were registered after the last theme switch.

lib/class-wp-rest-sidebars-controller.php

@@ -116,8 +116,10 @@ public function get_items_permissions_check( $request ) { // phpcs:ignore Variab
 	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
 	 */
 	public function get_items( $request ) {
+		retrieve_widgets();
+
 		$data = array();
-		foreach ( (array) wp_get_sidebars_widgets() as $id => $widgets ) {
+		foreach ( wp_get_sidebars_widgets() as $id => $widgets ) {
 			$sidebar = $this->get_sidebar( $id );
 
 			if ( ! $sidebar ) {
@@ -153,6 +155,8 @@ public function get_item_permissions_check( $request ) { // phpcs:ignore Variabl
 	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
 	 */
 	public function get_item( $request ) {
+		retrieve_widgets();
+
 		$sidebar = $this->get_sidebar( $request['id'] );
 
 		if ( ! $sidebar ) {