Issue #3629503: Reject button fails when an approval task is opened as a full page

In the Form Approval Flow example, clicking Reject and send back to Employee on a non-modal approval task leaves the page unchanged. No error appears on the page, but Drupal records a PHP error.

The rejection may have been processed before the response fails, leaving the user unsure whether the action succeeded.

Logs this error: TypeError: Drupal\Core\Render\MainContent\AjaxRenderer::renderResponse(): Argument #1 ($main_content) must be of type array, null given.