private void OnVisualPopupToolTipDisposed(object sender, EventArgs e)
{
// Unhook events from the specific instance that generated event
VisualPopupToolTip popupToolTip = (VisualPopupToolTip)sender;
popupToolTip.Disposed -= new EventHandler(OnVisualPopupToolTipDisposed);
// Not showing a popup page any more
_visualPopupToolTip = null;
}