JabRef does not shutdown

[koppor]

Originally reported at #11574 (comment).

Quick fix: Revert #11574

Longer fix: Investigate the lists of background tasks.

Discussion:

Removal of finished background tasks was added, because during summer, each indexed PDF was appearing as background task. That lead to thousands of tasks and lead to UI issues. Now, it seems that both AI (@InAnYan ) and Lucene (@LoayGhreeb) do a summary task. It would be OK to keep the finished tasks.

Currently, finished tasks cannot be removed by a user: controlsfx/controlsfx#1559

My proposal:

Investigate 5 hours either in fixing the exception in JabRef - or add the functionality to remove the finished tasks in TaskProgressView .

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• Refs JabRef doesn't quit InAnYan/jabref#178.
• Refs BackgroundTask interrupted when shutting down JabRef LoayGhreeb/jabref#6

[koppor]

When we go back to the full list - the PR controlsfx/controlsfx#1567 is a good thing to include.