To fix this issue, you must include "-ObjC" flag in "Other Linker Flags" found in your projects build settings.
UAirship library 3.x now makes use of Objective-C categories so this flag must be set to avoid the runtime exception.
Details on the issue can be found here:
https://developer.apple.com/library/mac/qa/qa1490/_index.html
Updated the docs:
http://docs.urbanairship.com/build/ios.html#build-settings http://docs.urbanairship.com/topic_guides/ios_migration.html#linker-flags