2023-September-21 - 2.6.14512

Fix NPE with UnitHelp. (#11977)

This fixes the NPE with Unit Help while preserving the functionality where the progress dialog stays shown until Unit Help is opened. (This is particularly noticeable on the Warcraft map where otherwise there's a 9s delay on my machine after the progress dialog is closed and before unit help is shown.)

The NPE was caused by my previous attempt at this doing Swing work on a background thread (which apparently worked fine on Mac, but not Windows). Instead, this PR reworks the logic with a more complicated solution that allows plumbing the expensive Swing work to be done to the background runner, so that it can run it before it closes the progress dialog.