Add definition.

templates/carousel/theme.inc

@@ -14,6 +14,8 @@ function template_preprocess_views_bootstrap_carousel_plugin_rows(&$vars) {
   $view = &$vars['view'];
 
   foreach ($vars['options'] as $id => $field) {
-    $vars[$id] = $view->field[$field]->theme($vars['row']);
+    if ($field) {
+      $vars[$id] = $view->field[$field]->theme($vars['row']);
+    }
   }
 }