Glade error

Hello,
I get an error when running a program which uses a graphical interface 
built with glade.
This is the error:

(marionnet.byte:6041): libglade-CRITICAL **: glade_xml_build_interface: 
assertion `wid != NULL' failed
Fatal error: exception Failure("Glade error: 
GtkDialog:dialog_RELATIVIZATION is not accessible.")
Raised at file "pervasives.ml", line 22, characters 22-33
Called from file "gui.ml", line 617, characters 9-85
Called from file "gui/gui_menubar_MARIONNET.ml", line 356, characters 17-49
Called from unknown location
Called from unknown location

I don't understand this error. I thought that it was caused by a missing 
identifier or an identifier mismatch, but I checked the window (dialog) 
identifiers in all files (including "gui.ml" and 
"gui/gui_menubar_MARIONNET.ml") and they appear to be the same.
What is this exception actually telling me?

Thanks in advance

-- 
Gianluca