How repeatable is this? Is the device coming out of sleep mode? What is on the screen when the App goes into the background?
Within the ViewController you can try registering the NSNotification event, UIApplicationWillResignActiveNotification
When the App is going into the background, calling cancel on any MKMapSnapshotter
If that doesn’t work, you could attempt more drastic measures, like hiding views, just to identify if they are related to the crash.