I've seen a couple of other applications which produce the message: Failed to load module "topmenu-gtk-module"
This appears to be a problem with dist-upgrading ubuntu-mate-default-settings from 16.04 to 18.04. The upgrade installs /etc/X11/Xsession.d/80mate-environment but does not remove the older /etc/X11/Xsession.d/99mate-environment. Deleting the latter fixed the problem.
I've seen a couple of other applications which produce the message: Failed to load module "topmenu- gtk-module"
This appears to be a problem with dist-upgrading ubuntu- mate-default- settings from 16.04 to 18.04. The upgrade installs /etc/X11/ Xsession. d/80mate- environment but does not remove the older /etc/X11/ Xsession. d/99mate- environment. Deleting the latter fixed the problem.