In some case original is not set and should be checked

Problem/Motivation

When saving programmatically a node containing file fields, I some occasion, I got a WSOD due to the following code in FileFieldItemList.php::postSave()

      $original = $entity->original;
      if ($original->hasTranslation($langcode)) {
        ...
        }
      }

Steps to reproduce

Difficult to say... maybe linked to the revision feature

Proposed resolution

Check if $original is set