Issue #3281740 by attisan: Fixed crash on entity tab showing custom form mode.

   * {@inheritdoc}

   */

  public function buildContent(EntityInterface $target_entity) {

    return $this->entityFormBuilder->getForm($target_entity, $this->configuration['form_mode']);

    $entity_type = $target_entity->getEntityType();

    $operation = $this->configuration['form_mode'];

    if ($entity_type->getFormClass($operation) === NULL) {

      // Hotpatch in the default handler for the given form mode as a workaround

      // for a core bug.

      // @see https://www.drupal.org/project/drupal/issues/2530086.

      // @todo: remove once the core issue is fixed.

      $entity_type->setFormClass($operation, $entity_type->getFormClass('default'));

    }

    return $this->entityFormBuilder->getForm($target_entity, $operation);

  }

}