When I tried to remove a selected entry from a group I got the follow error message "Uncaught exception occurred in Thread[#36, JavaFX Application Tread,5,main]. So, I was unable to remove selected entry from a group and also the message does not go away. So, basically I had to restart JabRef again.
For reference I am using JabRef 5.9 on macOS Monterey.
Please find attached the entire error message and a print screen of my JabRef 5.9. The entry editor was not opened when I tried to remove the selected group entry. The same behaviour happened using JabRef 5.7.