Fix NPE when tab is close during load

[koppor]

When a large library is loaded and the tab is close during load, there is an exception:

java.lang.NullPointerException: Cannot invoke "javafx.scene.control.TabPane.getSelectionModel()" because the return value of "org.jabref.gui.LibraryTab.getTabPane()" is null
	at org.jabref@100.0.0/org.jabref.gui.LibraryTab.setDatabaseContext(LibraryTab.java:264)
	at org.jabref@100.0.0/org.jabref.gui.LibraryTab.onDatabaseLoadingSucceed(LibraryTab.java:243)

This PR fixes that - and also adds some NPE checks at close (to enable the tab being closed despite NPEs)

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.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.