fetchJuso method Null safety

Implementation

fetchJuso(String keyword, int pageNumber) async {
  Map<String, String> parameter = {
    "confmKey": jusoKey,
    "currentPage": "$pageNumber",
    "countPerPage": "10",
    "keyword": keyword,
    "resultType": "json",
  };

  String queryString = Uri(queryParameters: parameter).query;

  try {
    JusoResult jusoResult = await _jusoService.searchJuso(queryString);

    if (jusoResult.jusoList.isEmpty && jusoResult.common.errorCode == '0') {
      throw ErrorModel(statusCode: 0, error: -101, message: '검색 결과가 없습니다.');
    } else if (jusoResult.common.errorCode != '0') {
      throw ErrorModel(statusCode: 0, error: 0, message: jusoResult.common.errorMessage);
    }
    if (pageNumber == 1) _jusoList.clear();
    _jusoList.addAll(jusoResult.jusoList);
    _fetcher.sink.add(_jusoList);
  } catch (e) {
    _fetcher.sink.addError(e);
  }
}