Bugfix: UI elements accessed outside of EDT (ImporterAgent)

This would be a bugfix for issue: https://www.openmicroscopy.org/qa2/qa/feedback/8002/

The ClassCastException is completely misleading, as the underlieing problem is the access of UI elements of the ImporterUI's ClosedTabbedPane outside of the Event Dispatching Thread.

To Test: Start several parallel image imports, then randomly switch between the import tabs.