You are calling GridView.RenderControl(htmlTextWriter)
, hence the page raises an exception that a Server-Control was rendered outside of a Form.
You could avoid this execption by overriding VerifyRenderingInServerForm
public override void VerifyRenderingInServerForm(Control control)
{
/* Confirms that an HtmlForm control is rendered for the specified ASP.NET
server control at run time. */
}
See here and here.