When starting applications in the mate desktop environment on Ubuntu 20.04 a warning showed up in the terminal:
Gtk-Message: Failed to load module “topmenu-gtk-module“
A mate community forum thread explained the cause:
An old settings file from Ubuntu 18.04 tried to load the module. I had a new and an old file:
root@sybo:/etc/X11/Xsession.d# ls -lah -rw-r--r-- 1 root root 564 Apr 2 2020 80mate-environment -rw-r--r-- 1 root root 530 Jan 19 20:47 99mate-environment
For some reason the old "99" file had not been removed during the upgrade from Ubuntu 18.04 to 20.04, causing the warnings.