Hi

I wrote following code. Not sure whether it is correct way or not.

1. This code works good under Forms -> Prevalue Sources
2. It is showing all values from web api 3.Then I am able to add dropdown on form also. No error
3. When I am try to add form on the page. I am getting following exception

Any idea, how to solve this issue. I am using Clean Starter Kit with umbraco 8

public class ExperimentDataSource : FieldPreValueSourceType {

    [Setting("Url", Description = "Enter the url to post to", View = "TextField")]
    public string Url
    {
        get;
        set;
    }

    [Setting("Value to Display", Description = "Enter the url to post to", View = "TextField")]
    public string ListOfValues
    {
        get;
        set;
    }

    public ExperimentDataSource()
    {
        Id = new Guid("74557737-FE48-4911-9FC2-952315B85B63");
        Name = "Read From Web Service";
        Description = "This is experiement";

    }

    public override List<PreValue> GetPreValues(Field field, Form form)
    {
        List<PreValue> result = new List<PreValue>();


        using (WebClient http = new WebClient())
        {
            http.Headers.Add(HttpRequestHeader.ContentType, "application/json");

            try
            {
                var json = new WebClient().DownloadString(Url);
                dynamic dynJson = JsonConvert.DeserializeObject(json);
                foreach (var item in dynJson)
                {
                    PreValue pv = new PreValue();
                    pv.Id = item.ID;
                    pv.Value = item[ListOfValues].ToString();
                    result.Add(pv);
                }

            }
            catch (Exception ex)
            {
                string err = String.Format("There was a problem sending the Record with unique id '{0}' from the Form '{1}' with id '{2}' to the URL Endpoint '{3}' with the method 'POST'");

            }
        }



        return result;
    }

    public override List<Exception> ValidateSettings()
    {

        List<Exception> exceptions = new List<Exception>();
        if (Url.Trim().Length < 1)
        {
            exceptions.Add(new Exception("Url value must be supplied."));
        }
        return exceptions;
    }
}

exception System.Web.HttpException (0x80004005): Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'. ---> System.NullReferenceException: Object reference not set to an instance of an object. at Umbraco.Forms.Web.Models.FormViewModel.<>c.