refs #19547 [finc] bugfix in CartController

* now shows error message in lightbox

module/finc/src/finc/Controller/CartController.php

@@ -117,7 +117,7 @@ class CartController extends \VuFind\Controller\CartController
         }
 
         // finc specific: dont redirect in layout context for performance reasons - page reloads after closing of lighbox anyway
-        if ($this->params()->fromPost('layout', $this->params()->fromQuery('layout')) === 'lightbox') {
+        if ($flashNamespace !== 'error' && $this->params()->fromPost('layout', $this->params()->fromQuery('layout')) === 'lightbox') {
             $view = $this->createViewModel($this->params()->fromPost());
             $view->setTemplate('default/flash-message');
             return $view;