AutoComplte Extender is not working

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Ajax

Hi,
I am here simply trying to use Ajax:AutoCompleteExtender control from AjaxToolKit.
But its not working properly
The used codes are shown below

<asp:TextBox runat="server" Width="110px" ID="txtSeat"
                           ontextchanged="txtPNR_TextChanged" AutoPostBack="true"
                           AutoCompleteType="Search"></asp:TextBox>
                       <Ajax:AutoCompleteExtender runat="server" ID="extender1"
                           TargetControlID="txtSeat" UseContextKey="True" servicepath="SeatAutoFill.asmx" ServiceMethod="GetCompletionList" MinimumPrefixLength="2" CompletionInterval="1000" EnableCaching="true"></Ajax:AutoCompleteExtender>

[System.Web.Services.WebMethod]
        [System.Web.Script.Services.ScriptMethod]
        public static List<string> GetCompletionList1(string prefixText, int count)
        {
            string seat="";
            List<string> list = new List<string>();
            n_DBConnection dbcon = new n_DBConnection();

            DataTable dt = new DataTable();
            dt = dbcon.getDatatable("select * from Exam_SeatMappin where Seat='"+prefixText+"'+'%'");
            if (dt.Rows.Count > 0)
            {
                seat=(dt.Rows[0]["Seat"].ToString());
            }

            list.Add(seat);
            return list;
        }

AjaxToolKit is working properly b/c I used number of control (like popup control, Calender control, etc.)
On browser console it shown following exception
1.)Uncaught Sys.ScriptLoadFailedException: Sys.ScriptLoadFailedException: The script 'http://localhost:443/ScriptResource.axd?d=Wa9Weah3h2mr083ggFSzFhAAFlWTL088I…Icu3eB1fSG9UwKBSbKkV4b41gDrOPa841waJBpX_xbmjAi4Jy6fTD_dKcQWgQykH_XFHWo_WrG...<omitted>...)
2.)Uncaught TypeError: Cannot read property '_notified' of null

Please help me get out from it