As far as I am aware dolfin-x is not built for ubuntu-18.04, see: Install dolfinx on Ubuntu server and WSL - #4 by dparsons
I am also uncertain if the PPA allows you to install the complex support version of dolfinx, @dparsons would know more about this.
To run a working version of the code, I would suggest the following:
docker run -ti -e DISPLAY=$DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix -v $(pwd):/root/shared -w /root/shared --rm --shm-size=512m dolfinx/dolfinx
source dolfinx-complex-mode
which should run fine and produce correct plots in paraview.