Error Populating Cascading DropdownList inside a gridview using Web Service in ASP.NET

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="available_services.aspx.cs" MasterPageFile="~/Forms/Home.Master"  Inherits="HospitalManagementSystem.Forms.available_services" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
 <script type="text/javascript" src="scripts/jquery-1.10.1.min.js"></script>

<div>

    <ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1"  runat="server">
    </ajaxToolkit:ToolkitScriptManager>

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField HeaderText="Name" DataField="ContactName" />
            <asp:TemplateField HeaderText="Country">
                <ItemTemplate>
 <asp:DropDownList ID="ddlCountries" runat="server" Width="150">
                    </asp:DropDownList>
                    <cc1:CascadingDropDown ID="cdlCountries" TargetControlID="ddlCountries" PromptText="Select Country" PromptValue=""
                        ServicePath="ServiceCS.asmx" ServiceMethod="GetCountries"
                         runat="server" Category="Country" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="City">
                <ItemTemplate>
                    <asp:DropDownList ID="ddlCities" runat="server" Width="150">
                    </asp:DropDownList>
                    <cc1:CascadingDropDown ID="cdlCities" TargetControlID="ddlCities" PromptText="Select City" PromptValue=""
                        ServicePath="ServiceCS.asmx" ServiceMethod="GetCities"  runat="server"
                        Category="City" ParentControlID="ddlCountries" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
 </div>
</asp:Content>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

using System.Data;
using System.Configuration;
using System.Data.SqlClient;
using System.Web.Services;
using AjaxControlToolkit;


namespace Testing.Forms
{

public partial class available_services : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack) //Only do this first time it's loaded
        {
            BindMajorHeadDropdown();
            GridView1.DataSource = GetData("SELECT TOP 10 ContactName, Country FROM Customers");
            GridView1.DataBind();                              
        }
    }

private DataSet GetData(string query)
    {
        string conString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        SqlCommand cmd = new SqlCommand(query);
        using (SqlConnection con = new SqlConnection(conString))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;

                sda.SelectCommand = cmd;
                using (DataSet ds = new DataSet())
                {
                    sda.Fill(ds);
                    return ds;
                }
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.Services;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using AjaxControlToolkit;



namespace Testing
{

/// <summary>
/// Summary description for ServiceCS
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]

// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]

public class ServiceCS : System.Web.Services.WebService
{

    [WebMethod]
    public CascadingDropDownNameValue[] GetCountries(string knownCategoryValues)
    {
        string query = "SELECT DISTINCT Country FROM Customers";
        List<CascadingDropDownNameValue> countries = GetData(query);
        return countries.ToArray();
    }

    [WebMethod]
    public CascadingDropDownNameValue[] GetCities(string knownCategoryValues)
    {
        string country = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues)["Country"];
        string query = string.Format("SELECT DISTINCT City FROM Customers WHERE Country = '{0}'", country);
        List<CascadingDropDownNameValue> cities = GetData(query);
        return cities.ToArray();
    }

    private List<CascadingDropDownNameValue> GetData(string query)
    {
        string conString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        SqlCommand cmd = new SqlCommand(query);
        List<CascadingDropDownNameValue> values = new List<CascadingDropDownNameValue>();
        using (SqlConnection con = new SqlConnection(conString))
        {
            con.Open();
            cmd.Connection = con;
            using (SqlDataReader reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    values.Add(new CascadingDropDownNameValue
                    {
                        name = reader[0].ToString(),
                        value = reader[0].ToString()
                    });
                }
                reader.Close();
                con.Close();
                return values;
            }
        }
    }
}
}