Forms Data Controls :: Throw The Exception Back To Page_Load Error: ArgumentNullException Was Unhandled By User Code

I'm trying to throw the exception back to Page_Load but the throw statement causes error: ArgumentNullException was unhandled by user code. How can that be fixed? Also, I can't see Label1.Text displayed on the page after the ArgumentNullException occurs because the page is not re-load. If there is no exception, it's fine to not reload the page. How can I see the Label1.Text displayed?

protected void Page_Load(object sender, EventArgs e)
{
try
{
...do something here...
}
catch (ArgumentNullException ae)
{
System.Diagnostics.Debug.Write(ae.Message);
Label1.Text = ae.Message;
Label1.Style.Add("visibility", "visible");
}
catch (Exception ex)
{
...do something here...
}
}
protected void ImageButton1_Click(object sender, ImageClickEventArgs e)............................