Warning in Xcode console: Couldn't read values in CFPrefsPlistSource

I have enabled an App Group in my App and the Widget Extension. I use it to share my UserDefaults. Every time the app starts I now get the following error message in the Xcode console:

Couldn't read values in CFPrefsPlistSource<0x303034510> (Domain: group.XX.XXXX.XXXX, User: kCFPreferencesAnyUser, ByHost: Yes, Container: (null), Contents Need Refresh: Yes): Using kCFPreferencesAnyUser with a container is only allowed for System Containers, detaching from cfprefsd

The shared UserDefaults itself works without problems.

Any ideas how I could get rid of this warning?