Promotion entities throw an exception when toUrl() is called on them

Reproduce steps:

1. Install drupal8.6 and group;1.x-dev and commerce
2. Create group_content:commerce_promotion plugin use these code
   

   /**
    * Provides a content enabler for nodes.
    *
    * @GroupContentEnabler(
    *   id = "group_promotion",
    *   label = @Translation("Group promotion"),
    *   description = @Translation("Adds promotion to groups both publicly and privately."),
    *   entity_type_id = "commerce_promotion",
    *   entity_access = TRUE,
    *   reference_label = @Translation("Title"),
    *   reference_description = @Translation("The title of the promotion to add to the group"),
    * )
    */
   class GroupPromotion extends GroupContentEnablerBase {
   }
   

3. Create a group_content-commerce_promotion, when click save, the site crash
   

   The website encountered an unexpected error. Please try again later.</br></br><em class="placeholder">Drupal\Core\Entity\Exception\UndefinedLinkTemplateException</em>: No link template &#039;canonical&#039; found for the &#039;commerce_promotion&#039; entity type in <em class="placeholder">Drupal\Core\Entity\Entity-&gt;toUrl()</em> (line <em class="placeholder">224</em> of <em class="placeholder">core/lib/Drupal/Core/Entity/Entity.php</em>). <pre class="backtrace">group_content_entity_submit(Array, Object)
   call_user_func_array(&#039;group_content_entity_submit&#039;, Array) (Line: 111)
   Drupal\Core\Form\FormSubmitter-&gt;executeSubmitHandlers(Array, Object) (Line: 51)
   Drupal\Core\Form\FormSubmitter-&gt;doSubmitForm(Array, Object) (Line: 589)
   Drupal\Core\Form\FormBuilder-&gt;processForm(&#039;commerce_promotion_add_form&#039;, Array, Object) (Line: 318)
   Drupal\Core\Form\FormBuilder-&gt;buildForm(&#039;commerce_promotion_add_form&#039;, Object) (Line: 48)
   Drupal\Core\Entity\EntityFormBuilder-&gt;getForm(Object, &#039;add&#039;, Array) (Line: 361)
   Drupal\group\Entity\Controller\GroupContentController-&gt;createForm(Object, &#039;group_promotion&#039;)
   call_user_func_array(Array, Array) (Line: 123)
   Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber-&gt;Drupal\Core\EventSubscriber\{closure}() (Line: 582)
   Drupal\Core\Render\Renderer-&gt;executeInRenderContext(Object, Object) (Line: 124)
   Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber-&gt;wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
   Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber-&gt;Drupal\Core\EventSubscriber\{closure}() (Line: 151)
   Symfony\Component\HttpKernel\HttpKernel-&gt;handleRaw(Object, 1) (Line: 68)
   Symfony\Component\HttpKernel\HttpKernel-&gt;handle(Object, 1, 1) (Line: 67)
   Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap-&gt;handle(Object, 1, 1) (Line: 57)
   Drupal\Core\StackMiddleware\Session-&gt;handle(Object, 1, 1) (Line: 47)
   Drupal\Core\StackMiddleware\KernelPreHandle-&gt;handle(Object, 1, 1) (Line: 99)
   Drupal\page_cache\StackMiddleware\PageCache-&gt;pass(Object, 1, 1) (Line: 78)
   Drupal\page_cache\StackMiddleware\PageCache-&gt;handle(Object, 1, 1) (Line: 41)
   Drupal\jsonapi\StackMiddleware\FormatSetter-&gt;handle(Object, 1, 1) (Line: 47)
   Drupal\Core\StackMiddleware\ReverseProxyMiddleware-&gt;handle(Object, 1, 1) (Line: 38)
   Drupal\webprofiler\StackMiddleware\WebprofilerMiddleware-&gt;handle(Object, 1, 1) (Line: 52)
   Drupal\Core\StackMiddleware\NegotiationMiddleware-&gt;handle(Object, 1, 1) (Line: 23)
   Stack\StackedHttpKernel-&gt;handle(Object, 1, 1) (Line: 665)
   Drupal\Core\DrupalKernel-&gt;handle(Object) (Line: 19)
   </pre>
   

The error caused here:
group.module

  if ($entity->access('view')) {
    $form_state->setRedirectUrl($entity->toUrl());
  }

Let's see the code of $entity->toUrl() (\Drupal\Core\Entity\Entity).

  public function toUrl($rel = 'canonical', array $options = []) {
    if ($this->id() === NULL) {
      throw new EntityMalformedException(sprintf('The "%s" entity cannot have a URI as it does not have an ID', $this->getEntityTypeId()));
    }

    // The links array might contain URI templates set in annotations.
    $link_templates = $this->linkTemplates();

    // Links pointing to the current revision point to the actual entity. So
    // instead of using the 'revision' link, use the 'canonical' link.
    if ($rel === 'revision' && $this instanceof RevisionableInterface && $this->isDefaultRevision()) {
      $rel = 'canonical';
    }

    if (isset($link_templates[$rel])) {
      $route_parameters = $this->urlRouteParameters($rel);
      $route_name = "entity.{$this->entityTypeId}." . str_replace(['-', 'drupal:'], ['_', ''], $rel);
      $uri = new Url($route_name, $route_parameters);
    }
    else {
      $bundle = $this->bundle();
      // A bundle-specific callback takes precedence over the generic one for
      // the entity type.
      $bundles = $this->entityTypeBundleInfo()->getBundleInfo($this->getEntityTypeId());
      if (isset($bundles[$bundle]['uri_callback'])) {
        $uri_callback = $bundles[$bundle]['uri_callback'];
      }
      elseif ($entity_uri_callback = $this->getEntityType()->getUriCallback()) {
        $uri_callback = $entity_uri_callback;
      }

      // Invoke the callback to get the URI. If there is no callback, use the
      // default URI format.
      if (isset($uri_callback) && is_callable($uri_callback)) {
        $uri = call_user_func($uri_callback, $this);
      }
      else {
        throw new UndefinedLinkTemplateException("No link template '$rel' found for the '{$this->getEntityTypeId()}' entity type");
      }
    }

    // Pass the entity data through as options, so that alter functions do not
    // need to look up this entity again.
    $uri
      ->setOption('entity_type', $this->getEntityTypeId())
      ->setOption('entity', $this);

    // Display links by default based on the current language.
    // Link relations that do not require an existing entity should not be
    // affected by this entity's language, however.
    if (!in_array($rel, ['collection', 'add-page', 'add-form'], TRUE)) {
      $options += ['language' => $this->language()];
    }

    $uri_options = $uri->getOptions();
    $uri_options += $options;

    return $uri->setOptions($uri_options);
  }

If the entity don't have 'canonical' or bundle's uri_callback, then an Exception will be throw out.