Entry preview: error when saving new styles

[wujastyk]

JabRef 5.1--2020-04-21--dc1e339
Linux 5.3.0-46-generic amd64
Java 14.0.1

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

Steps to reproduce the behavior:

1. Options/Preferences/Entry Preview
2. add some journal styles
3. hit "save". Error popup, as shown below

and

Log File

Paste an excerpt of your log file here

[Siedlerchr]

Duplicate of #6198