Fixing bug in screensaver – xUbuntu 18.04 LTS

Last fresh install of the desktop version of xUbuntu 18.04 (3rd release) turned out to have a bug in the lock behavior: whenever the screen/session was supposed to lock, the computer stopped instead. The bug comes from xfce screensaver. Installing

sudo apt install xscreensaver

let xscreensaver takes the precedent over xfce screensaver and solves the issue.