C# JArray - System.NullReferenceException Error Object reference not set to an instance of an object

I'm trying to display the simple element of the array or value. How can I do that? Below is my JSON string(from python pandas):

{
    "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"
            ]
        }
    }
}

Below code of C# is the query that I'm using:

            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);
            ........
            ........
            ........

And I got the following error on JArray columns = (JArray)jsonFeed["data"]["schema"]["fields"];

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

I am very new to C#, is that an array,domain or url problem, or anything else? How can I do that? thank you so much for any advice.