You may be tempted to use a check like this one to determine whether the
widget has yet to be destroyed:

	(XtWindowToWidget(XtDisplay(toplevel), XtWindow(widget)) == widget)

However, this is not a robust solution. It depends on assumptions about how
Xt uses memory allocation and how that memory allocation works; in addition,
it assumes that no subsequent widgets are created with the same handle and
window id.

In addition, XtIsWidget() simply checks whether an existing, valid object
is a widget or a gadget. It can't be used to check for current existence.

In general, you should track destruction, possibly by using
XtNdestroyCallback callbacks.