After upgrading from Ubuntu MATE 16.04 to 18.04 I am seeing this error message while starting virtually every GUI program (e.g. caja, mate-terminal, mate-calc, pluma, gvim, soffice, google-chrome, firefox).
I also see the error recorded in syslog:
$ grep "topmenu-gtk-module" /var/log/syslog
Sep 1 10:25:38 mypc wnck-applet[2662]: Failed to load module "topmenu-gtk-module"
Sep 1 10:25:38 mypc notification-ar[2676]: Failed to load module "topmenu-gtk-module"
Sep 1 10:25:38 mypc mate-indicator-[2674]: Failed to load module "topmenu-gtk-module"
Sep 1 10:25:38 mypc clock-applet[2675]: Failed to load module "topmenu-gtk-module"
Sep 1 10:25:38 mypc mate-drivemount[2684]: Failed to load module "topmenu-gtk-module"
Sep 1 10:25:38 mypc /usr/bin/python[2663]: Failed to load module "topmenu-gtk-module"
Sep 1 11:26:45 mypc mate-notificati[11945]: Failed to load module "topmenu-gtk-module"
On https://packages.ubuntu.com I see that topmenu-gtk packages are not available for bionic.
So, how to fix this issue on 18.04?