loadNext method Null safety

Implementation

@override
Future<List<User>> loadNext() async {
  if (loading) throw QueryInProgressError();
  if (!hasNext) return [];

  loading = true;

  final sdk = SendbirdSdk().getInternal();
  final res = await sdk.api.send<PollVoterListQueryResponse>(
    PollOptionGetListVotersRequest(
      channelType: params.channelType,
      channelUrl: params.channelUrl,
      pollId: params.pollId,
      pollOptionId: params.pollOptionId,
      token: token,
      limit: limit,
    ),
  );

  loading = false;
  token = res.next;
  hasNext = res.next != '';
  return res.voters;
}