Hi Mike,
Yes, this is a known issue with the current 1.8.4 version. This problem has been completely fixed in version 1.8.5, which will be released very soon. So I ask for a bit of patience.
A workaround is to perform a reset of application settings. right-click (or Ctrl-click) the PixInsight.app application bundle (or the corresponding icon on the dock), then keep the Ctrl key pressed while you select Open from the context menu. Accept the confirmation dialog. This will clear all application settings and quitting the application will be much faster. As I've said, this won't happen with version 1.8.5. Sorry for the trouble.