Hi Agnès,
I've just tried going through the same process with JMatch (Ubuntu 20.04
LTS, Wine 5.0.3) and I don't get the error. So it doesn't seem to be
anything to do with closing the app and cancelling save-changes. Was
there anything unusual about the file that you were working on?
If I could find a reliable way to reproduce it, I could probably fix it.
Cheers,
Martin