'NSUnknownKeyException', reason: '[<CardGameViewController 0x717cf00> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key card.'
crash for NSUnknownKeyException solved.

modified main.m to output exception call stack.

(removed all buttons and added new ones from scratch. it seemed in storyboard Referencing Outlet Connections was automatically added to two targets. maybe Xcode bug?)