Populate ComboBox from JSON Object

Question

Hello, I have below Json

[
    {
        "customerid": 1,
        "name": "ABS",
        "email": "true",
        "reference": "2016-03-21T10:44:32+03:00",
        "status": 0,
        "dateupdated": "2017-12-07T04:56:05.583"
    },
    {
        "customerid": 2,
        "name": "BMS",
        "email": "true",
        "reference": "2016-03-13T19:17:54+03:00",
        "status": 0,
        "dateupdated": "2017-12-07T04:56:05.757"
    },
    {
        "customerid": 3,
        "name": "XYA",
        "email": "true",
        "reference": "2016-03-13T19:17:54+03:00",
        "status": 0,
        "dateupdated": "2017-12-07T04:56:05.757"
    }
]

In my windows form I create class API.cs

class API
    {

    }

    public class RootObject
    {
        public int customerid { get; set; }
        public string name { get; set; }
        public string email { get; set; }
        public DateTime reference { get; set; }
        public int status { get; set; }
        public DateTime dateupdated { get; set; }
    }

I need to populate combo box customer_cb with "name".  I use DataContractJsonSerializer 

WebClient client = new WebClient();
  string json = client.DownloadString(url);
using (var ms = new MemoryStream(Encoding.Unicode.GetBytes(json)))
                {
                    DataContractJsonSerializer deserializer = new DataContractJsonSerializer(typeof(RootObject));
                    RootObject obj = (RootObject)deserializer.ReadObject(ms);

                    // Assign data to combo box
                    foreach(var name in obj.name)
                    {
                        ComboboxItem item = new ComboboxItem();
                        item.Text = name.ToString();
                        customer_cb.Items.Add(item);
                    }
                }

When I run the application I get error

Object Reference not set to an instance of an object

Please help.