gnome-panel broken: Gtk-CRITICAL **: file gtksignal.c: line 724 ...

[Jim Cline <jcline hep physics mcgill ca>]

I recently upgraded my debian system (using apt-get upgrade), unfortunately 
breaking several programs including gnome panel.
Starting panel gives the following messsages:

Gtk-WARNING **: invalid cast from (NULL) pointer to `GtkObject'
Gtk-CRITICAL **: file gtksignal.c: line 724 (gtk_signal_connect): assertion 
`object != NULL' failed.

I assume the latter is connected to the panel's failure to load any applets.  I 
was suspicious that there might be a conflict between the debian gtk libraries 
and some which at one point I installed manually, but
I have cleaned all the latter out of my system, as far as I can tell.
Any ideas would be appreciated, as I've tried everything I could think of.

thanks--Jim