lesson type sometimes disappears in the words lesson

[Marten de Vries]

The lesson type combo box is sometimes empty in the words lesson.

[Marten de Vries (marten-de-vries)]

Found the cause: settings depend on path names and those are different on my development system. (Sometimes my dev 'install' uses ~/.openteacher/store.json, and sometimes the Ubuntu package 'install' does.) For normal users, this is not a problem, it might only happen when upgrading. But, since the fallback (the default) is pretty easy and it can be 'fixed' by just selecting a lesson type once again, I don't consider this worth changing stuff. Updating status accordingly.