MessageBoxExt calls a method name ShowAlert: with the IOSAppDelegate as a receiver, but the IOSAppDelegate class does not have that method implemented which results in a crash whenever MessageBoxExt is called.
The line responsible for the call is:
https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Runtime/Core/Private/IOS/IOSPlatformMisc.cpp#L237
Looks like it may have been a bad merge or some such.
This is happening on a shipping build compiled from Xcode.
Reproducible: always.
Engine Version: 4.7.5