Exception when closing a library with groups

[aqurilla]

This fixes #9997 by adding null checks for the root node of groupTree.
No further exceptions are observed in the log when closing a library with groups

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]

LGTM just add a quick note in the changelog. I don't know if it was present in 5.9 or not, but in any way helpful to have it listed as fixed

[koppor]

In main, I sometimes had to click the close button at a tab twice. This is now solved, too.