After updating lib_gtk, gtk started reporting warning lines like these:
(rumbax:20026): Gtk-CRITICAL **: 18:50:36.197: gtk_widget_get_visible: assertion 'GTK_IS_WIDGET (widget)' failed
(rumbax:20026): Gtk-CRITICAL **: 18:50:36.214: gtk_widget_hide: assertion 'GTK_IS_WIDGET (widget)' failed
not a problem, but it's annoying.
I compiled libgtk2 and libgtk3 without "introspection" support and the problem disappeared.
I don't know exactly what "introspection" does, but for sure it doesn't work
This topic is here for those who are experiencing the same problem.