Fix undefined index in request

main/inc/lib/CoursesAndSessionsCatalog.class.php

@@ -1998,7 +1998,7 @@ class CoursesAndSessionsCatalog
         $action = isset($action) ? Security::remove_XSS($action) : $requestAction;
         $searchTerm = isset($_REQUEST['search_term']) ? Security::remove_XSS($_REQUEST['search_term']) : '';
         $keyword = isset($_REQUEST['keyword']) ? Security::remove_XSS($_REQUEST['keyword']) : '';
-        $searchTag = $_REQUEST['search_tag'] ? Security::remove_XSS($_REQUEST['search_tag']) : '';
+        $searchTag = isset($_REQUEST['search_tag']) ? Security::remove_XSS($_REQUEST['search_tag']) : '';
         $languageSelect = isset($_REQUEST['course_language']) ? Security::remove_XSS($_REQUEST['course_language']) : '';
 
         if ($action === 'subscribe_user_with_password') {