check for principaluri

			$uid = $row['uid'];

			$sender = substr($row['attendee'], 7);

			$organizer = $row['organizer'];

			$recipient = substr($organizer, 7);

			$principalUriOfOrganizer = $this->principalBackend->findByUri($organizer, 'principals/users');

			if ($principalUriOfOrganizer === null) {

				$this->logger->warning('Principal URI not found for organizer', ['organizer' => $organizer]);

				return $response;

			}

			$query2 = $this->db->getQueryBuilder();

			$query2

				->select('co.*')