JabRef version JabRef 5.0-dev--snapshot--2019-01-21--master--40d049e08 on Ubuntu 16.04

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

Steps to reproduce the behavior:

1. Open any database
2. Select an entry and copy & past the same entry, so that a duplicate is created
3. Click Quality => Find duplicates
4. The duplicate window is totally empty, no boxes or text showing up. Only the buttons at the bottom show. (If I try the same thing in v 4.3.1 it works as expected; two duplicates are showing in the left and right halves for the window.)

No errors are showing up in the console when starting from command line.