Background Updates: Avoid a PHP warning during theme update checks when there are multiple theme directories registered. Fixes #27815

Built from https://develop.svn.wordpress.org/trunk@28137


git-svn-id: http://core.svn.wordpress.org/trunk@27968 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/class-wp-upgrader.php

@@ -1962,7 +1962,7 @@ class WP_Automatic_Updater {
 				break;
 			case 'theme':
 				$upgrader = new Theme_Upgrader( $skin );
-				$context  = get_theme_root( $item );
+				$context  = get_theme_root( $item->theme );
 				break;
 			case 'translation':
 				$upgrader = new Language_Pack_Upgrader( $skin );