system crashes when exiting from console 18.04

1) ubuntu 10.04
2) No package
3) On exiting from console, I expected to get a new login prompt.
4) Ubuntu switches back to graphics mode frozen. Screen looks normal but with no arrow.
   Keyboard is frozen. I can only stop by using the power switch.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: crash (not installed)
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat May 19 09:07:01 2018
InstallationDate: Installed on 2018-05-06 (12 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: crash
UpgradeStatus: No upgrade log present (probably fresh install)