Parsing JSON says Object Reference not set to instance of an object xamarin.forms

Question

I am trying to parse the JSON but it is giving me error as Object Reference not set to instance of an object what i am trying is JSON:

    [
    {"Product":"Product 1",
    "Origins":
    [
        {"Origin":"XXX QQ",
        "Details":   
        [
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"},
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"}
        ]
        },
        {"Origin":"ADAMI Oil",
        "Details":   
        [
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"},
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"}
        ]
        }       
    ]
    },
    {"Product":"Product 2",
    "Origins":
    [
        {"Origin":"YYY pp",
        "Details":   
        [
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"},
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"}
        ]
        },
        {"Origin":"PQR CCC",
        "Details":   
        [
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"},
            {"Seller":"Name","Seller_Rate":"256","Saller_QTY":"20","Buyer":"Name","Buyer_Rate":"256","Buyer_QTY":"20"}
        ]
        }       
    ]
    }
]

My Object Class that holds the JSON Values:

public class Products
{
    public List<Origins> ListOrigin { get; set; }
    public string PRODUCT { get; set; }
}
public class Origins
{
    public string Origin { get; set; }
    public List<Details> ListDetails { get; set; }
}
public class Details
{
    public string Seller { get; set; }
    public string Seller_Rate { get; set; }
    public string Saller_QTY { get; set; }
    public string Buyer { get; set; }
    public string Buyer_Rate { get; set; }
    public string Buyer_QTY { get; set; }
}

call to web-service and parsing :

string responsejson = await client.GetStringAsync(Constants.url_getproducts);
        res = JsonConvert.DeserializeObject<List<Products>>(responsejson);
        try
        {
            foreach (Products p in res)
            {
                Log.Debug("Login1", p.PRODUCT);

                foreach (Origins o in p.ListOrigin)
                {
                    Log.Debug("Login1", o.Origin);
                }
            }
        }
        catch (Exception e){
            Log.Debug("Login e", e.Message);
            Log.Debug("Login e", e.StackTrace);
        }

Here when i am trying to parse it in foreach, Log.Debug("Login1", p.PRODUCT); this gives Product properly but for Origins it says

Object reference not set to an instance of an object.

One Solution i got is:

try
        {
            root = JsonConvert.DeserializeObject<List<Products>>(responsejson);
            Log.Debug("Login",Convert.ToString(root.Count));
            for(int i=0;i<root.Count;i++)
            {
                Log.Debug("Login", "=============================");
                Log.Debug("Login", root[i].PRODUCT);
                for(int j=0;j<root[i].Origins.Count;j++)
                {
                    Log.Debug("Login","   "+root[i].Origins[j].Origin);
                    for(int k=0;k<root[i].Origins[j].Details.Count;k++)
                    {
                        Log.Debug("Login", "       Buy " + root[i].Origins[j].Details[k].Buyer+" BUY_QTY "+ root[i].Origins[j].Details[k].Buyer_QTY);

                    }
                }
                Log.Debug("Login", "=============================");
            }
        }