iOS8でUIImagePickerControllerをpresentViewControllerする際、表題のワーニングが出ます。
どうやらiOS8のバグのよう。
stackoverflow--iOS 8 Snapshotting a view that has not been rendered results in an empty snapshot
"You can even download Apple's Using UIImagePickerController sample app, run it, and it will generate the same error out of the box." - 2014 Sep 24
とあるので、一旦無視で。