Issue #2713985 by larowlan: Not all entities have a _self href use UUID instead in dependency graph

          // Decode the file contents.

          $decoded = $this->serializer->decode($contents, 'hal_json');

          // Get the link to this entity.

          $self = $decoded['_links']['self']['href'];

          $item_uuid = $decoded['uuid'][0]['value'];

          // Throw an exception when this URL already exists.

          if (isset($file_map[$self])) {

          if (isset($file_map[$item_uuid])) {

            $args = array(

              '@href' => $self,

              '@first' => $file_map[$self]->uri,

              '@uuid' => $item_uuid,

              '@first' => $file_map[$item_uuid]->uri,

              '@second' => $file->uri,

            );

            // Reset link domain.

            $this->linkManager->setLinkDomain(FALSE);

            throw new \Exception(SafeMarkup::format('Default content with href @href exists twice: @first @second', $args));

            throw new \Exception(SafeMarkup::format('Default content with uuid @uuid exists twice: @first @second', $args));

          }

          // Store the entity type with the file.

          $file->entity_type_id = $entity_type_id;

          // Store the file in the file map.

          $file_map[$self] = $file;

          $file_map[$item_uuid] = $file;

          // Create a vertex for the graph.

          $vertex = $this->getVertex($self);

          $this->graph[$vertex->link]['edges'] = [];

          $vertex = $this->getVertex($item_uuid);

          $this->graph[$vertex->id]['edges'] = [];

          if (empty($decoded['_embedded'])) {

            // No dependencies to resolve.

            continue;

          }

          // Here we need to resolve our dependencies;

          // Here we need to resolve our dependencies:

          foreach ($decoded['_embedded'] as $embedded) {

            foreach ($embedded as $item) {

              $edge = $this->getVertex($item['_links']['self']['href']);

              $this->graph[$vertex->link]['edges'][$edge->link] = TRUE;

              $uuid = $item['uuid'][0]['value'];

              $edge = $this->getVertex($uuid);

              $this->graph[$vertex->id]['edges'][$edge->id] = TRUE;

            }

          }

        }

   */

  protected function getVertex($item_link) {

    if (!isset($this->vertexes[$item_link])) {

      $this->vertexes[$item_link] = (object) array('link' => $item_link);

      $this->vertexes[$item_link] = (object) array('id' => $item_link);

    }

    return $this->vertexes[$item_link];

  }