Used when a form needs to return some kind of a \Symfony\Component\HttpFoundation\Response object, e.g., a \Symfony\Component\HttpFoundation\BinaryFileResponse when triggering a file download. If you use self::setRedirect() or self::setRedirectUrl(), it will be used to build a \Symfony\Component\HttpFoundation\RedirectResponse and will populate this key.
Type: \Symfony\Component\HttpFoundation\Response|null
protected $response;
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Form!FormState.php/property/FormState::response/8.1.x