Show data on map Jquery

Question

I try to plot data from gridview on map like this

[WebMethod]
        public static string mapgriddata(string FID,string ObtID)
        {

            try
            {
                string result = "";
                FID = HttpContext.Current.Session["FID"].ToString();
                ObID = HttpContext.Current.Session["ObID"].ToString();
                Entities w = new Entities();
                List<spgetdata_ffid_Result> gt = w.spgetdata_ffid(FFID).ToList();

                DataTable dt = new DataTable();
                dt.Columns.Add("NO", typeof(string));
                dt.Columns.Add("Longitude", typeof(float));
                dt.Columns.Add("Latitude", typeof(float));


                foreach (var t in gt)
                {

                    dt.Rows.Add(t.No, t.Longitude, t.Latitude);


                }

                string No = HttpContext.Current.Session["No"].ToString();

                float Latitude = Convert.ToSingle(HttpContext.Current.Session["Latitude"]);
                float Longitude = Convert.ToSingle(HttpContext.Current.Session["Longitude"]);


                result = DataSetToJSON(dt);
                return result;

            }
            catch (Exception)
            {
                throw new Exception();


            }

        }

jquery

<script type="text/javascript">
        $(function () {
            var FID = '<%=Session["FID"]%>';
            var ObID = '<%=Session["ObID"]%>';
            var No = '<%=Session["No"]%>';

            var longi = '<%=Session["Longitude"]%>';
            var latit = '<%=Session["Latitude"]%>';


            var obj = {};
            obj.FID = FID;
            obj.ObID = ObID;
            obj.No = No;
            obj.longi = longi;
            obj.latit = latit;

            getdata(obj);
            return falase;

        });
        $(function getdata(obj) {
            $.ajax({
                type: "POST",
                url: "home.aspx/mapgriddata",
                contentType: "application/json;charset=utf-8",
                data: (data.FFID, data.ObID),
                datatype: "json",
                async: true,
                cache: false,

                success: function (result) {
                    window.onload = function () {
                        alert("map2");
                        debugger;
                        var latlng = new google.maps.LatLng(24.0895898, 67.0998546);
                        debugger;
                        var mapOptions = {

                            center: new google.maps.LatLng(markers[0].lat, markers[0].lng),
                            zoom: 8,
                            center: latlng,
                            mapTypeId: google.maps.MapTypeId.ROADMAP
                        };
                        debugger;
                        var infoWindow = new google.maps.InfoWindow();
                        debugger;
                        var map = new google.maps.Map(document.getElementById("map_canvas"), mapOptions);
                        debugger;
                        for (i = 0; i < obj.length; i++) {
                            var data = obj[i]
                            var myLatlng = new google.maps.LatLng(obj.latlng);
                            var marker = new google.maps.Marker({
                                position: myLatlng,
                                map: map,

                            });
                            debugger;
                            (function (marker, obj) {
                                google.maps.event.addListener(marker, "click", function (e) {
                                 infoWindow.setContent(obj.FID,obj.No,obj.latit,obj.longi);
                                    infoWindow.open(map, obj);
                                });
                            })(marker, data);
                        }
                    }
                },
                error: function (error) {
                    alert(error);
                }
            });
        });




</script>

 page load 

protected void Page_Load(object sender, EventArgs e)
      {
        mapgriddata(Convert.ToString(Session["FID"]),Convert.ToString(Session["ObID"]));


          }

      }

 

but this show error

Object reference not set to an instance of an object.