I have a jquery JStree and after selecting node I use ajax to post the selected node to server. In code behind , a method get this nodeID and everything works correctly.

$("#tree1").bind("select_node.jstree", function (e, data) {
   if (jQuery.attr(data.rslt.obj[0], "href"))
   { 
      window.location = jQuery.attr(data.rslt.obj[0], "#div2"); }
      if (jQuery.attr(data.rslt.obj[0], "id"))
       {
         // alert(jQuery.attr(data.rslt.obj[0], "id"));
         document.getElementById('<%= hdFolderID.ClientID %>').value =
         jQuery.attr(data.rslt.obj[0], "id"); } $.ajax({
         type: "POST", 
         url: "FolderPage.aspx/GetSelectedFolder", 
         contentType: "application/json; charset=utf-8", 
         data: "{'FolderID': '" + jQuery.attr(data.rslt.obj[0], "id") + "'}",
         dataType: "json", 
         success: function (response) { } }); });

code behind:

 [WebMethod(EnableSession = true)] 
public
 static void GetSelectedFolder(int FolderID)
 {
       new FolderPage().ShowFilesForSelectedFolder(FolderID);
 
}

In ShowFilesForSelectedFolder method I use this FolderID to populate other details on a gridview.But I can not bind the data in gridview and get this eror: Object reference not set to an instance of an object.after ajax POST to set a value for any page control I get this error.

 gridview1.DataSource = results; 
 gridview1
.DataBind();

How can I access to page controls?