loadNext method Null safety

Implementation

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

  loading = true;

  final sdk = SendbirdSdk().getInternal();
  final res = await sdk.api.send<MessageSearchQueryResponse>(
    MessageSearchRequest(
      keyword: keyword,
      channelUrl: channelUrl,
      customType: channelCustomType,
      reverse: reverse,
      exactMatch: exactMatch,
      beforeIndex: beforeIndex,
      afterIndex: afterIndex,
      token: token,
      targetUserIds: targetUserIds,
      startAt: messageTimestampFrom,
      endAt: messageTimestampTo,
      sortField: messageSearchQueryOrderEnumMap[order],
      limit: limit,
      advanced: advancedQuery,
      targetFields: targetFields,
    ),
  );

  loading = false;
  token = res.next;
  hasNext = res.hasNext;
  return res.results;
}