Here's a full crash report: crash_report.txt
mdhorda
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In case this is helpful for anyone, I got this response from Apple:
Looking through your crash logs the cause appears to be related to the use of gradient layers. This is either causing a memory allocation error or some other problem in your app. Please review your use of gradient layers.
Thanks for you response. The issue ended up being that we had mistakenly created 2 UITabBarController instances and moved the child viewcontrollers over to the one that wasn't being displayed. The crash occurred when a tab bar item was tapped and the associated viewcontroller was referencing the other UITabBarController instance.