[Gtk-sharp-list] Medsphere.Widgets.FBox crashes with "must provide a protected or public IntPtr ctor"

[Adam Tauno Williams]

I've create a widget that extends the very cool Medsphere FBox

  public class PhoneTable : Medsphere.Widgets.FBox
  {
    public PhoneTable(Whitemice.ZOGI.Company company) : base()
    {
	...
    }
  }

And it works for awhile.  Then it crashes with -

Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to
handle the exception.
GLib.MissingIntPtrCtorException: GLib.Object subclass
Whitemice.ZOGI.GtkWidgets.PhoneTable must provide a protected or public
IntPtr ctor to support wrapping of native object handles.
  at GLib.ObjectManager.CreateObject (IntPtr raw) [0x00000] 
  at GLib.Object.GetObject (IntPtr o, Boolean owned_ref) [0x00000] 
  at Gtk.Container.Forall_cb (IntPtr container, Boolean
include_internals, IntPtr cb, IntPtr data) [0x00000] 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e,
Boolean is_terminal)
   at Gtk.Container.Forall_cb(IntPtr container, Boolean
include_internals, IntPtr cb, IntPtr data)
   at GLib.ToggleRef.g_object_remove_toggle_ref(IntPtr ,
GLib.ToggleNotifyHandler , IntPtr )
   at GLib.ToggleRef.Free()
   at GLib.Object.PerformQueuedUnrefs()
   at GLib.Timeout+TimeoutProxy.Handler()
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Whitemice.Consonance.Consonance.Main(System.String[] args)

It seems like it happens as soon as an FBox has been disposed.  Is there
something different with the Medsphere widgets?  I've bene using Gtk#
for years and never had to deal with an IntPtr.