trait EntityReferenceFieldTrait in Multiversion 8

trait EntityReferenceFieldTrait {
  public function preSave() {
    if ($this
      ->hasNewEntity()) {

      // As part of a bulk or replication operation there might be multiple
      // parent entities wanting to auto-create the same reference. So at this
      // point this entity might already be saved, so we look it up by UUID and
      // map it correctly.
      // @see \Drupal\relaxed\BulkDocs\BulkDocs::save()
      if ($this->entity
        ->isNew()) {
        $uuid = $this->entity
          ->uuid();
        $uuid_index = \Drupal::service('multiversion.entity_index.factory')
          ->get('multiversion.entity_index.uuid');
        if ($uuid && ($record = $uuid_index
          ->get($uuid))) {

          /** @var \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager */
          $entity_type_manager = \Drupal::service('entity_type.manager');
          $entity_type_id = $this->entity
            ->getEntityTypeId();

          // Now we have to decide what revision to use.
          $id_key = $entity_type_manager
            ->getDefinition($entity_type_id)
            ->getKey('id');

          // If the referenced entity is a stub, but an entity already was
          // created, then load and use that entity instead without saving.
          if ($this->entity->_rev->is_stub && is_numeric($record['entity_id'])) {
            $this->entity = $entity_type_manager
              ->getStorage($entity_type_id)
              ->load($record['entity_id']);
          }
          elseif (!$this->entity->_rev->is_stub) {
            $this->entity->{$id_key}->value = $record['entity_id'];
            $this->entity
              ->enforceIsNew(FALSE);
            $this->entity
              ->save();
          }
        }
        else {
          $this->entity
            ->save();
        }
      }

      // Make sure the parent knows we are updating this property so it can
      // react properly.
      if (empty($this->entity) && !empty($record['entity_id'])) {
        $id = $record['entity_id'];
      }
      else {
        $id = $this->entity
          ->id();
      }
      $this->target_id = $id;
    }
    if (!$this
      ->isEmpty() && $this->target_id === NULL) {
      $this->target_id = $this->entity
        ->id();
    }
  }

}