FlutterHttps未处理异常：无效参数

我想在列表视图中显示数据（例如硬编码）

当我想获得价值时，它不起作用

I/flutter ( 9640): [{"No":0,"interest":"0.00","balance":"11,000,000,000.00","principal":"0.00","Installment":"0.00","Status":true},{"No":1,"interest":"110,000,000.00","balance":"0.00","principal":"11,000,000,000.00","Installment":"11,110,000,000.00","Status":true}]
I/flutter ( 9640): true
E/flutter ( 9640): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Invalid argument(s)
E/flutter ( 9640): #0      _StringBase.+ (dart:core-patch/string_patch.dart:272:57)

这是 FetchData 异步

 List<LoanModel> _loanmodel = <LoanModel>[];

  Future<LoanModel> _fetchData() async {
    setState(() {
      loading = true;
    });
    final response =
    await http.get("http://192.168.0.23/Api/loansimulation.php?periodtime=" + periodtime + "&interestpermonth=" + interestpermonth + "&loanamountrequest=" +loanamountrequest); //with hardcode working fine
    if (response.statusCode == 200) {
      final data = jsonDecode(response.body);
      setState(() {
        for (Map i in data) {
          _loanmodel.add(LoanModel.fromJson(i));
        }
        loading = false;
      });
    }
  }

如何从 API 中获取 periodtime、interestpermonth 和 Loanamountrequest 的值并根据 flutter 中的输入值获取值？