TestMediaFilterController.php

<?php

namespace Drupal\media_test_embed\Controller;

use Drupal\filter\FilterFormatInterface;
use Drupal\media\Controller\MediaFilterController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

/**
 * Controller to allow testing of error handling of Media Embed in text editors.
 */
class TestMediaFilterController extends MediaFilterController {
    
    /**
     * {@inheritdoc}
     */
    public function preview(Request $request, FilterFormatInterface $filter_format) {
        if (\Drupal::state()->get('test_media_filter_controller_throw_error', FALSE)) {
            throw new NotFoundHttpException();
        }
        return parent::preview($request, $filter_format);
    }

}