Update means of generating LinkHeader URL.

... allow for catching exceptions for entities without canonical
URLs.

src/EventSubscriber/LinkHeaderSubscriber.php

@@ -9,6 +9,7 @@ use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\islandora\IslandoraUtils;
+use Drupal\Core\Entity\Exception\UndefinedLinkTemplateException;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Response;
@@ -205,7 +206,14 @@ abstract class LinkHeaderSubscriber implements EventSubscriberInterface {
         // Headers are subject to an access check.
         if ($referencedEntity->access('view')) {
 
-          $entity_url = $this->utils->getEntityUrl($referencedEntity);
+          try {
+            $entity_url = $this->utils->getEntityUrl($referencedEntity);
+          }
+          catcH (UndefinedLinkTemplateException $e) {
+            // Not all referencable entities can generate canonical URLs, for
+            // example: block entities.
+            continue;
+          }
 
           // Taxonomy terms are written out as
           // <url>; rel="tag"; title="Tag Name"

src/IslandoraUtils.php

@@ -563,15 +563,17 @@ class IslandoraUtils {
    *
    * @return string
    *   The entity URL.
+   *
+   * @throws \Drupal\Core\Entity\Exception\UndefinedLinkTemplateException
+   *   Should the given entity not specify a "canonical" template.
    */
   public function getEntityUrl(EntityInterface $entity) {
     $undefined = $this->languageManager->getLanguage('und');
-    $entity_type = $entity->getEntityTypeId();
-    return Url::fromRoute(
-      "entity.$entity_type.canonical",
-      [$entity_type => $entity->id()],
-      ['absolute' => TRUE, 'language' => $undefined]
-    )->toString();
+    return $entity->toUrl('canonical', [
+        'absolute' => TRUE,
+        'language' => $undefined,
+      ])
+      ->toString();
   }
 
   /**