• [ X ] I have tested the latest master version from http://builds.jabref.org/master/ and the problem persists

JabRef 5.0-dev--snapshot--2018-09-13--master--e83680f18
Linux 4.4.0-135-generic amd64
Java 1.8.0_181

Steps to reproduce:

1. Click on two entries
2. Right-click and choose "Merge entries".

The merge window is shown, and JabRef is frozen - it must be killed from the command line.
There are no particular log messages.