JabRef version 5.2, 5.3 and 100.0.0 (commit 0dd4de7) on Linuxmint-20.1 and LinuxMint-19.3.

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

Steps to reproduce the behavior:

1. Go to "Options|Preferences|Entry preview|Customised preview style|Edit"
2. Start typing new format elements into the text box, e.g.: "\begin{doi}, DOI: \doi\end{doi}" after the "...\end{page}",
3. The exception is triggered and pops up a window, disrupting the work:
   

The edit still can be completed, but the box pops up sevral times, interfering with the editing process....