UI API called from background thread

Suddenly out of nowhere I'm getting this error when I try to run the game from Xcode. Nothing has changed since the last time I ran the game in Xcode other then the computer rebooted overnight.

Does anyone have a hint at what could be causing this to get tripped?

 

I've discovered that this happens only when Mac App Store Validation is checked in Project Settings -> Player.

Archiving and uploading seems to produce something that Apple can review but I'm also not sure if my inability to test with it checked is causing some failures with other features that rely on IAP functionality.