Apt is a command line utility for install, update, remove and other operations. It manage dep packages in Ubuntu, Debian and distributions that based on it. It consists of most used commands apt-get and apt-cache but differs the default options. Apt was created for interactive use. Most of the commands of apt utility should be executed with sudo premissions.

In the last (at that moment) version of Lubuntu 19.04 desktop environment was moved to LXQT. In looks fine, but the role of screensaver still occupied by xscreensaver application with it's awful window to enter the credentials. I've already have a post about theming this window https://16rom.com/ru/blog/temizaciya-xsreensaver .

You should delete it immediately after OS installation: 

Started session for anonymous user in mixed mode cause PDOException when user tries to log in. PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'h0c...' for key 'PRIMARY': UPDATE {sessions}