AlterBodySubscriber error when response HTML does not contain ___storybook_wrapper

Problem/Motivation

We have a custom module that calls ContainerAwareEventDispatcher::dispatchRequest() to dispatch a fake request.
This request is caught by AlterBodySubscriber::alter() which fails with this error:

DOMNode::appendChild(): Argument #1 ($node) must be of type DOMNode, null given

Steps to reproduce

    $fakeResponse = new HtmlResponse();
    $fakeResponse->setContent($html);

    $event = new ResponseEvent(
      \Drupal::service('http_kernel'),
      \Drupal::requestStack()->getMainRequest()->duplicate(),
      HttpKernelInterface::SUB_REQUEST,
      $fakeResponse
    );
    $this->eventDispatcher->dispatch($event, KernelEvents::RESPONSE);

Proposed resolution

AlterBodySubscriber should make sure ___storybook_wrapper exists before trying to use it.