Can't preview a new node

To reproduce:

1. Add a disqus comment field to a content type, eg: article
2. Go to the page to a new node, at eg: /node/add/article
3. Enter any required fields, like title
4. Click "Preview" to preview the node

The following error is generated:

Drupal\Core\Entity\EntityMalformedException: The "node" entity cannot have a URI as it does not have an ID in Drupal\Core\Entity\Entity->toUrl() (line 184 of core/lib/Drupal/Core/Entity/Entity.php).
Drupal\disqus\Plugin\Field\FieldFormatter\DisqusFormatter->viewElements(Object, 'en') (Line: 85)
Drupal\Core\Field\FormatterBase->view(Object, NULL) (Line: 264)
Drupal\Core\Entity\Entity\EntityViewDisplay->buildMultiple(Array) (Line: 308)
Drupal\Core\Entity\EntityViewBuilder->buildComponents(Array, Array, Array, 'default') (Line: 29)
Drupal\node\NodeViewBuilder->buildComponents(Array, Array, Array, 'default') (Line: 251)
Drupal\Core\Entity\EntityViewBuilder->buildMultiple(Array) (Line: 208)
Drupal\Core\Entity\EntityViewBuilder->build(Array)
call_user_func(Array, Array) (Line: 384)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 200)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 231)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 577)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 232)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 122)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 95)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object) (Line: 116)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 62)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 62)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 103)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 82)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 55)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 637)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

The cause of this is Disqus trying to render comments when the entity has no ID (is not yet saved). I think the correct behavior would be to just render nothing in that case, or maybe some sort of placeholder "disqus comments would go here".

A workaround for now is to disable access to the field for unsaved entities:

function mysite_deploy_entity_field_access($operation, FieldDefinitionInterface $field_definition, AccountInterface $account, FieldItemListInterface $items = NULL) {
  if ($field_definition->getType() == 'disqus_comment') {
    $entity = $items->getEntity();
    if (empty($entity->id())) {
      return AccessResult::forbidden();
    }
  }

  return AccessResult::neutral();
}