I found the solution here: https://techoverflow.net/2018/06/03/how-to-fix-freecad-no-module-named-webgui-on-ubuntu-18-04/
Obviously this person is aware of this bug, but has quite reasonably decided that Debian's response is hopeless, and the solution is to uninstall the Debian offering and go straight to the source, the FreeCAD ppa. Can't say I disagree.
However, it was the first hit using Google. Some users are never going to see this bug report, and will just follow the instructions. A.K.A. you are losing users due to inferior quality product. Better not to ship a FreeCAD package at all than ship a broken one, IMO.
-D.
