I think I’ve solved this: I had an entry under /etc/yunohost/domains which had no default_app: set. That appears to be the NoneType in question: having populated that, regenerating works.
I did check prior to this and yunohost domain info <mydomain> shows the app correctly, but it has to be in the file too.
After that, upgrading the package works again. So I’ve re-ran sudo apt-get dist-upgrade and let that finish, which it has done successfully. After rebooting things were fine.