Add null checks in GroupChangeViewModel

[lenhard]

This avoids the NPEs in #3526. However, I can't get the groups side pane to repaint and depict the modified groups tree. I've tried different ways via calling the GroupsSidePane or setting selected groups in the StateManager, but no success.

@tobiasdiez How do I force the GroupSidePane to display a changed group tree?

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• 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• Check documentation status (Issue created for outdated help page at help.jabref.org?)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?

[tobiasdiez]

Sadly updates to the group tree in general are ignored, see e.g. #3189. Normally, one would like that all changes are automatically propagated using the event bus / javafx binding, but for some reason this does not work (sorry, I forgot the details).

[lenhard]

Ok, thanks for clarifying. I am not sure what to do with this PR then. Under the hood it works (avoids the exceptions), but all of this is not visible to the user. Right now it requires a restart of the application.

[tobiasdiez]

Is the view updated when you switch between databases? This always worked for me so far. I would say merge this PR here (since it fixes the NPE) and comment in the above issue that there are more update problems. I'll have a look at this problem, but not right now as this week is kind of a hell for me.

[lenhard]

Yes, it is updated when switching between databases, e.g. all groups are removed and the all entries group is displayed when I remove the group statement. A rename of the group in the file doesn't carry over, so there are probably deeper problems.

I still have the problem that it tends to display the groups tree of the wrong database, but at least the change is reflected somehow.

[Siedlerchr]

Can this be merge now?

[lenhard]

Sure, go ahead.