##Do not activate Xenial repositories on Bionic! It may completely break the system.
Your error message looks very similar to the other thread.
The error about "topmenu-gtk-module" will get fixed if you remove (or move) one file:
sudo mv /etc/X11/Xsession.d/99mate-environment ~
(it came from 16.04 LTS installation, it is not actual for 18.04 LTS).