Fix hang when exporting at the CLI

[koppor]

Fixes #10380

There were two issues:

• NPE when getting the main table preferences.
• Hanging after a successful export.

The former was solved by using the right getter (e4ea759 (#10383)), the latter was solved by shutting down the file update monitor 18a3cd3 (#10383) (refs https://github.com/JabRef/jabref-issue-melting-pot/issues/290).

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

init with empty list?

[Siedlerchr]

looks good so far, but would need testing with the binary.

[koppor]

looks good so far, but would need testing with the binary.

Test was done at #10380 (comment).

I wonder whether we could simply do System.exit(0); instead doing a proper shut down after proper processing. -- I currently don't see any disadvantages there, but maybe, I overlook something?

[koppor]

I tried it with System.exit(0); at 231b3a9 (#10383).

[github-actions]

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[Siedlerchr]

the 60s shutdown is normally only the timeout if the tasks were not shutdown before:

jabref/src/main/java/org/jabref/gui/Globals.java

Lines 118 to 130 in a0e2564

	public static void shutdownThreadPools() {
	TASK_EXECUTOR.shutdown();
	if (fileUpdateMonitor != null) {
	fileUpdateMonitor.shutdown();
	}
	JabRefExecutorService.INSTANCE.shutdownEverything();
	}
	public static void stopBackgroundTasks() {
	Telemetry.shutdown();
	Unirest.shutDown();
	}