I know this is a very old issue, but I was just having this issue, and searches for a solution found a lot of people asking the same question, but no solutions. Hopefully this will help someone else…

I eventually figured out that my reference to the document picker was a local variable that was going out of scope after the picker is displayed. I believe when the view is dismissed, the picker is garbage collected before the callback occurs. I added a strong reference to the picker, and no longer get that error message, and the callbacks work correctly.