Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
2016-07-21 11:37:53.438 ws[7431:2987625] Warning: Attempt to present
<UIImagePickerController: 0x145011000> on <MainViewController:
0x144536ff0> whose view is not in the window hierarchy!