Export excel problem in asp.net

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ASP.NET

i am getting this error on excel sheet..
System.InvalidOperationException: RegisterForEventValidation can only be called during Render();


here my code

protected void btnexport_Click(object sender, ImageClickEventArgs e)
{
try
{

Response.ClearContent();
Response.Buffer = true;
Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "Planwise Report.xls"));
Response.ContentType = "application/ms-excel";
StringWriter sw = new StringWriter();
HtmlTextWriter htw = new HtmlTextWriter(sw);

btnview_Click(this, null);
GridView1.AllowPaging = false;
//Change the Header Row back to white color
GridView1.HeaderRow.Style.Add("background-color", "#FFFFFF");
//Applying stlye to gridview header cells
for (int i = 0; i < GridView1.HeaderRow.Cells.Count; i++)
{
GridView1.HeaderRow.Cells[i].Style.Add("background-color", "#507CD1");
}
int j = 1;
//This loop is used to apply stlye to cells based on particular row
foreach (GridViewRow gvrow in GridView1.Rows)
{
gvrow.BackColor = Color.White;
if (j <= GridView1.Rows.Count)
{
if (j % 2 != 0)
{
for (int k = 0; k < gvrow.Cells.Count; k++)
{
gvrow.Cells[k].Style.Add("background-color", "#EFF3FB");
}
}
}
j++;
}
GridView1.RenderControl(htw);
Response.Write(sw.ToString());

Response.Flush();

}
catch (Exception ex)
{
Response.Write(ex.ToString());
}
finally
{
Response.End();

}
}
public override void VerifyRenderingInServerForm(Control control)
{
/* Verifies that the control is rendered */
}

whats my mistake..?