loadNext method Null safety

Implementation

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

  loading = true;

  final sdk = SendbirdSdk().getInternal();
  final res = await sdk.api.send<OperatorListQueryResponse>(
    OperatorListRequest(
      token: token,
      limit: limit,
      channelType: channelType,
      channelUrl: channelUrl,
    ),
  );

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