The Phase 6 vocabulary training constantly, repeatedly, shows an annoying error message as an overlay over the app:
“Developer warning for package de.phase6.freeversion.beta. Failed to post notification on channel “null”. See log for details.”
Buttons do work even if covered by the error message, but you can’t read what’s written on them, so button pressing must be done blindly.
@lucaweiss: As this issue got reported for the calendar a while back, and it got fixed … FPoOS: com.android.calendar failed to post notification on channel "null" (#61) · Issues · Products / Android 10 on the FP2 · GitLab … Does the Phase 6 developer have to fix this, or is there something the user can do?
Still present on the latest update 10.
Sounds a bit like this as the package name reads .[…].beta. 
I see you already opened an issue, let’s link to it for others reading along …
com.android.calendar isn’t labelled beta, and from the issue tracker we don’t know how the issue got fixed for it .
The name part ‘beta’ is only visible if you look at the package name. Aurora/Playstore does not call it beta, nor do Klett or Phase 6. Phase 6 calls it version 9.1.2.!
My FPOOS is now 22.10.0-rel.0, the lateste, and the annoying error message is still there…
If you want to investigate more yourself you can use adb to have a look at the log and so search for more details.
The issue cleared with the last update of FPOOS Android 10. Probably the app still posts to that channel, but FPOOS won’t show annoying popups anymore…