Bugfix: UI elements accessed outside of EDT (ImporterAgent) (rebased onto dev_5_0)



This is the same as gh-2217 but rebased onto dev_5_0.

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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.