SystemConfigSubscriber.php

<?php

/**
 * @file
 * Contains \Drupal\system\SystemConfigSubscriber.
 */
namespace Drupal\system;

use Drupal\Core\Config\Config;
use Drupal\Core\Config\ConfigImporterEvent;
use Drupal\Core\Config\ConfigImporterException;
use Drupal\Core\Config\Context\ConfigContext;
use Drupal\Core\Config\Context\ContextInterface;
use Drupal\Core\Config\ConfigEvent;
use Drupal\Core\Config\StorageDispatcher;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * System Config subscriber.
 */
class SystemConfigSubscriber implements EventSubscriberInterface {

  /**
   * Implements EventSubscriberInterface::getSubscribedEvents().
   */
  static function getSubscribedEvents() {
    $events['config.importer.validate'][] = array(
      'onConfigImporterValidate',
      20,
    );
    return $events;
  }
  public function onConfigImporterValidate(ConfigImporterEvent $event) {
    $importer = $event
      ->getConfigImporter();
    $importList = $importer
      ->getStorageComparer()
      ->getSourceStorage()
      ->listAll();
    if (empty($importerList)) {
      throw new ConfigImporterException("This import will delete all your active configuration, I'm bailing out now.");
    }
  }

}