error export treeview to excel file

/******************************Error Message*****************************
RegisterForEventValidation can only be called during Render();
*****************************Export Code*******************************/
private void ExportTree()
    {
        
        Response.Clear();
        Response.Buffer = true;
        Response.ContentType = "application/vnd.ms-excel";
        Response.AddHeader("content-disposition","attachment;filename="+Ddl_Companies.SelectedItem.Text +"----"+ Ddl_TreeType.SelectedItem.Text+".xls");
        Response.Charset = "";
        //this.EnableViewState = false;
        System.IO.StringWriter oStringWriter = new System.IO.StringWriter();
        System.Web.UI.HtmlTextWriter oHtmlTextWriter = new System.Web.UI.HtmlTextWriter(oStringWriter);
        TreeNode tn=new TreeNode(Ddl_Companies.SelectedItem.Text +"----"+ Ddl_TreeType.SelectedItem.Text,"-1");
        Tv_Tree.Nodes.AddAt(0, tn);
        Tv_Tree.ExpandAll();
        Tv_Tree.ShowExpandCollapse = false;

        HtmlForm frm = new HtmlForm();
        Tv_Tree.Parent.Controls.Add(frm);
        frm.Attributes.Add("runat", "server");
        frm.Controls.Add(Tv_Tree);

        frm.RenderControl(oHtmlTextWriter);
        Response.Write(oStringWriter.ToString());
        Response.End();
    }