EntityFormModeAddForm.php

<?php

namespace Drupal\field_ui\Form;

use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

/**
 * Provides the add form for entity display modes.
 *
 * @internal
 */
class EntityFormModeAddForm extends EntityDisplayModeAddForm {
    
    /**
     * {@inheritdoc}
     */
    protected function prepareEntity() {
        $definition = $this->entityTypeManager
            ->getDefinition($this->targetEntityTypeId);
        if (!$definition->get('field_ui_base_route') || !$definition->hasFormClasses()) {
            throw new NotFoundHttpException();
        }
        $this->entity
            ->setTargetType($this->targetEntityTypeId);
    }

}