I am a new user to fenics and facing some problems at my first attempt. As you can see from the attached snapshot, whenever I try to run a script( demo_navier-stokes.py) which reads xml files, I get the same error message. Also plotting does not work. It gives an html address which is not working, however, solution files are saved into the directory correctly… I face the same issue on both docker containers in windows system and ubuntu versions (wsl on windows).
First make sure that you have used git-lfs to download the dolfin repository,as the xml files are not stored in the repo, as mentioned here: L-shape mesh generation with right element - #10 by Linda_Alzaben
To be able to plot the solution, you need to either save it to png import matplotlib.pyplot as plt; plt.savefig("file.png") instead of plt.show() or use Paraview to open output pvd or xdmf files.