Fix NPE in search bar

[tobiasdiez]

In #3471 a NPE was mentioned. I'm not sure which object was null (it should not be the scene, since this is just set and the css file should also be always found). Moreover, I could not reproduce the exception, so this fix is a shot in the blue.

---

• Change in CHANGELOG.md described
• Tests created for changes
• Screenshots added (for bigger UI changes)
• Manually tested changed features in running JabRef
• Check documentation status (Issue created for outdated help page at help.jabref.org?)
• If you changed the localization: Did you run gradle localizationUpdate?

[Siedlerchr]

Seems like this could be a LInux thing and related to our customjfx panel, see also the log in #3500

[lenhard]

Well, code-wise this are just minor changes, so you can merge if you think it's ready.

It would be better, though, if we knew that it actually fixes the problem.

[tobiasdiez]

I merge it now since it is still a slight improvement over the existing code. I'll have a look at the azure reports if a similar exception gets reported in the next days.