Revert/Delete user revision actions cause errors.

Closes #3455541

modules/user_revision_diff/src/Form/RevisionOverviewForm.php

@@ -216,8 +216,8 @@ class RevisionOverviewForm extends FormBase {
             '#theme' => 'username',
             '#account' => $revision->revision_uid->entity,
           );
-
-          $revision_date = $this->date->format($revision->revision_timestamp->value, 'short');
+          $revision_created_key = $revision->getEntityType()->getRevisionMetadataKey('revision_created');
+          $revision_date = $this->date->format($revision->get($revision_created_key)->value, 'short');
           // Use revision link to link to revisions that are not active.
           if ($vid != $default_revision) {
             $link = Link::fromTextAndUrl($revision_date, new Url('entity.user.revision', ['user' => $user->id(), 'user_revision' => $vid]));