reverted:
--- b/core/includes/batch.inc
+++ a/core/includes/batch.inc
@@ -15,7 +15,6 @@
  */
 
 use Drupal\Component\Utility\SafeMarkup;
-use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Timer;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Batch\Percentage;
@@ -24,7 +23,6 @@
 use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\RedirectResponse;
-use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 /**
  * Renders the batch processing page based on the current state of the batch.
@@ -45,7 +43,8 @@
   if (!$batch) {
     $batch = \Drupal::service('batch.storage')->load($request_id);
     if (!$batch) {
+      drupal_set_message(t('No active batch.'), 'error');
+      return new RedirectResponse(\Drupal::url('<front>', [], ['absolute' => TRUE]));
-      throw new NotFoundHttpException(String::format('Batch %id requested, but not found.', array('%id' => $request_id)));
     }
   }
   // Restore safe strings from previous batches.