app/bundles/NotificationBundle/Controller/AjaxController.php

<?php

namespace Mautic\NotificationBundle\Controller;

use Mautic\CoreBundle\Controller\AjaxController as CommonAjaxController;
use Mautic\CoreBundle\Controller\AjaxLookupControllerTrait;

class AjaxController extends CommonAjaxController
{
    use AjaxLookupControllerTrait;
}