Fix for multiple error messages in messed up source editor

[calixtus]

Fixes #5820

JabRef created a new instance of CodeArea everytime an entry was selected and bound multiple listeners to the CodeArea and to the ValidationStatus in bindToEntry. Refactored it a little bit, so the CodeArea stays persistent and the error messages are only displayed once.

Did some minor refactorings to the other Validators and to modules.info too, to avoid warning messages from the IDE, and fixed a minor mistake from a previous PR in dark.css.

No screenshot, as there is no visual change.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for bigger UI changes)
• Checked documentation: Is the information available and up to date? If not: Issue created at https://github.com/JabRef/user-documentation/issues.

[Siedlerchr]

Thanks for the fix. I don't know exactly what , but I remember we had issues with the code area when going up and down in the main table with an open entry editor. And also when importing?. I know that there was a reason why it was created new every time.
Maybe @tobiasdiez remembers

[tobiasdiez]

I cannot remember why the code area is re-created every time and I don't see any reason not to reuse it. I guess we simply took the original swing code, replaced the controls with javafx ones and didn't thought about reusing it. Let's try it and see if users report any problematic behavior.

The looks good to me. I've two small remarks/questions.

[calixtus]

This should do the trick now. Apparently JavaFX fires the focus event after loading the next entry and registering a listener does not mean, it is also run.

What I don't understand is the failing GUI test. It fails somehow during setting the test up trying to call getSourceString with an empty BibEntry.
It does not fail with a real entry open in JabRef, but only with the test. Should I modify the test?

[tobiasdiez]

The new fix looks good to!

I guess the test fails as currentEntry is null in https://github.com/JabRef/jabref/pull/5823/files#diff-e6eac13930d81c3c9c3ddc47d1b354a9R223
You can change https://github.com/JabRef/jabref/blob/master/src/test/java/org/jabref/gui/entryeditor/SourceTabTest.java#L81 to notifyAboutFocus.