public static function getSendTargetUser($activity) {
    // Get target account.
    if (isset($activity->field_activity_recipient_user) && !empty($activity->field_activity_recipient_user->target_id)) {
      $target_id = $activity->field_activity_recipient_user->target_id;
      $target_account = \Drupal::entityTypeManager()
        ->getStorage('user')
        ->load($target_id);
      return $target_account;
    }
  }