Setup – Dell Latitude 7490 running Ubuntu 18.04 Bionic and Dell WD15 USB-C dock.
Problem – system freezes when dock unplugged.
This problem started after updates. The solution found was to revert to the previous kernel (4.15.0-43-generic) from 4.15.0-44-generic. Did this by setting GRUB to remember the boot setting – change /etc/grub/grub with:
GRUB_DEFAULT=saved GRUB_SAVEDEFAULT=true
and run
update-grub
Then hit esc at the loading screen to get to the grub menu.