Fix opening of entry editor

[tobiasdiez]

Fixes #4762. The problem was that the NotificationPane was hiding the entry editor/preview.

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• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[matthiasgeiger]

Ahh... I also investigated the problem and came up with a similar solution, but had still some problems.

I tested your change now and there are also some issues remaining:

1. after creating a new database the screen is completely empty not showing the main table
2. after opening the preferences and saving, I cannot close the Entry editor anymore

... but I do not really understand why these aspects are currently failing

//Edit: I pushed my proposal to branch fix-4762 if you want to have a look

[Siedlerchr]

Lgtm. Just unsure if the show and edit is not called from a background thread somewhere. Could be the case that all are converted now

[tobiasdiez]

@matthiasgeiger sorry for the bad timing, I hope you didn't investigated too much time into this bug. You are right, these things should be fixed as well. Since I'll have on time to have a look at these issues in the next days but the bug addressed in this PR is mayor, I'll merge now but leave #4762 open as a reminder.