(附完整代码)求助懂 C#的大神， Excel 中显示 DataSet 数据集遇到 System.NullReferenceException 的报错是 NewtonJson Jarray 造成的吗

如题，我想在 Excel 中显示如下 JSON 数据集（ Python 写的 API 接口）

{
    "msg": "success",
    "state": 10000,
    "data": {
        "data": [
            {
                "index": 0,
                "test_1": 110,
                "test_2": "000001",
                "test_3": "CN",
                "test_4": "Bank",
                "test_5": 893,
                "test_6": 229
            }
        ],
        "schema": {
            "fields": [
                {
                    "type": "integer",
                    "name": "index"
                },
                {
                    "type": "string",
                    "name": "test_1"
                },
                {
                    "type": "string",
                    "name": "test_2"
                },
                {
                    "type": "number",
                    "name": "test_3"
                },
                {
                    "type": "number",
                    "name": "test_4"
                },
                {
                    "type": "number",
                    "name": "test_5"
                },
                {
                    "type": "string",
                    "name": "test_6"
                }
            ],
            "pandas_version": "0.20.0",
            "primaryKey": [
                "index"
            ]
        }
    }
}

下面是我 C#的数据集代码 TestDataset.cs:

using System;
using System.IO;
using System.Net;
using Newtonsoft.Json.Linq;
using ExcelDna.Integration;
using Excel = Microsoft.Office.Interop.Excel;
namespace Test_Atune
{
    public class Request
    {
        Excel.Application ExcelApp = (Excel.Application)ExcelDnaUtil.Application;
        public object GetDatasetFromUrl(string root, string path, string headval, Excel.Range excelAddress)
        {
            string historicalData= "{}";
            var webConnection = new WebClient();
            webConnection.Headers.Add("Host", headval);
            try
            {
                historicalData = webConnection.DownloadString(root+path);
            }
            catch (WebException ex)
            {
                string error_str = ex.Message;
                if (ex.Status.ToString() == "ConnectFailure" || ex.Status.ToString() == "Timeout")
                {
                    root = Constant.setURL_ROOT_COMMON(root);
                    try
                    {
                        historicalData= webConnection.DownloadString(root + path);
                    }
                    catch (WebException ex2)
                    {
                        return "";
                    }
                }
            }
            finally
            {
                webConnection.Dispose();
            }

            JObject jsonFeed = JObject.Parse(historicalData);
            Excel.Range range = excelAddress;
            JObject B = new JObject();
            JArray data = (JArray)jsonFeed["data"]["data"];
            JArray columns = (JArray)jsonFeed["data"]["schema"]["fields"];
            int rowNum = data.Count;
            int colNum = columns.Count;
            Excel.Range range_head = ExcelApp.Cells[range.Row + 1, range.Column];
            range_head = range_head.get_Resize(1, colNum);
            Excel.Range range_data = ExcelApp.Cells[range.Row + 2, range.Column];
            range_data = range_data.get_Resize(rowNum, colNum);
            // write header
            object[,] headerData = new object[1, colNum];
            for (int iCol = 0; iCol < colNum; iCol++)
            {
                headerData[0, iCol] = columns[iCol]["name"];
            }
            range_head.Value2 = headerData;

            // write data
            object[,] cellData = new object[rowNum, colNum];
            int iRow = 0;
            foreach (JObject jo in data)
            {
                var a = jo["test_code"];
                for (int iCol = 0; iCol < colNum; iCol++)
                {
                    if (columns[iCol]["test_1"].ToString() == "string")
                    {
                        cellData[iRow, iCol] = "'" + jo[columns[iCol]["name"].ToString()];
                    }
                    else
                    {
                        cellData[iRow, iCol] = jo[columns[iCol]["name"].ToString()];
                    }
                }
                iRow += 1;
            }
            range_data.Value2 = cellData;
            return "Total" + rowNum.ToString() + "cells";
        }
    }

}

请求 Python 接口的 C#代码 request.cs

using ExcelDna.Integration;
using Excel = Microsoft.Office.Interop.Excel;
namespace Test_Atune
{
    public class Dataset
    {
        public static string baseurl = Constant.URL_ROOT_COMMON;
        public static string headval = Constant.HEADVAL_COMMON;
        public static Request request = new Request();

        [ExcelFunction(Category = "test", IsMacroType = true, Description = "test dataset")]
        public static object TEST_DATASET(
                    [ExcelArgument(Description = "test_code")] string test_1,
                    [ExcelArgument(Description = "YYYYMMDD")] string test_2,
                    [ExcelArgument(Description = "YYYYMMDD")] string test_3
                    )
        {
            string parstr = @"/test_dataset/?" +
                @"test_1=" + test_1 +
                @"&test_2=" + test_2 +
                @"&test_3=" + test_3;
            ExcelReference caller = (ExcelReference)XlCall.Excel(XlCall.xlfCaller);
            Excel.Range rg = caller.ToPiaRange();
            return ExcelAsyncUtil.Run("TEST_DATASET",
                new object[] { parstr },
                () => request.GetDatasetFromUrl(Constant.URL_ROOT_COMMON, parstr, headval, rg));
        }
    }

}

但在这一行 JArray columns = (JArray)jsonFeed["data"]["schema"]["fields"];

我遇到了如下报错：

System.NullReferenceException HResult=0x80004003  message=Object reference not set to an instance of an object.

Debug 后得到如下结果，是否 NewtonJson Jarray 需要判断下是否为空值？数据集不太会判别，对 C#一窍不通，求大神指点一二，非常感谢 Name: historicalData, Value:"{}"; Name: jsonFeed,Vaule:Null; Name:B,Vaule:{{}}, Name:data, Value:"null"