The Viewer app is not the same as a Player, it only receives what is being displayed in Windows. It doesn't allow you to interact and is really not good for testing.
And, it won't show if there is an issue in iOS because the app is not actually running on it.
Good luck looking at the code.
My best guesses of what to look for are doing something with a sprite/image id for a sprite/image that has been deleted or never created. Or some index that is way out of range in an array and you are either using something that doesn't actually exist or have set something in the array and clobbered something in the app (since the index was outside of the array).
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master