Fixing PHP warning.

main/admin/course_add.php

@@ -31,7 +31,7 @@ $order_clause = api_sort_by_first_name() ? ' ORDER BY firstname, lastname' : ' O
 $table_user = Database :: get_main_table(TABLE_MAIN_USER);
 $sql = "SELECT user_id,lastname,firstname FROM $table_user WHERE status=1".$order_clause;
 // Filtering teachers when creating a course.
-if ($_configuration['multiple_access_urls']) {
+if (api_is_multiple_url_enabled()) {
     $access_url_rel_user_table= Database :: get_main_table(TABLE_MAIN_ACCESS_URL_REL_USER);
     $sql = "SELECT u.user_id,lastname,firstname FROM $table_user as u
             INNER JOIN $access_url_rel_user_table url_rel_user