Coding Standards: Use is_wp_error() in WP_Customize_Custom_CSS_Setting.

This brings more consistency with similar checks elsewhere in core.

Follow-up to [39350].

Props dilipbheda, mukesh27.
Fixes #63363.
Built from https://develop.svn.wordpress.org/trunk@60215


git-svn-id: http://core.svn.wordpress.org/trunk@59551 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/customize/class-wp-customize-custom-css-setting.php

@@ -197,9 +197,10 @@ final class WP_Customize_Custom_CSS_Setting extends WP_Customize_Setting {
 			)
 		);
 
-		if ( $r instanceof WP_Error ) {
+		if ( is_wp_error( $r ) ) {
 			return false;
 		}
+
 		$post_id = $r->ID;
 
 		// Cache post ID in theme mod for performance to avoid additional DB query.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.9-alpha-60213';
+$wp_version = '6.9-alpha-60215';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.