Fix removing button target when dialog gets closed.

This fix was derived from a rather generic crash log report so it is not possible to tell which button caused the crash. All buttons in the app received treatment similar to the suggested fix.

Exception Type: EXC_BAD_ACCESS Code: KERN_INVALID_ADDRESS at 0x1374697186
0com.apple.main-thread Crashed
0	 libobjc.A.dylib	 objc_msgSend + 15
1	 UIKit 	 -[UIApplication sendAction:to:from:forEvent:] + 70
2	 UIKit	 -[UIApplication sendAction:toTarget:fromSender:forEvent:] + 30
3	 UIKit	 -[UIControl sendAction:to:forEvent:] + 44
4	 UIKit	 -[UIControl(Internal) _sendActionsForEvents:withEvent:] + 502
5	 UIKit	 -[UIControl touchesEnded:withEvent:] + 488
6	 UIKit	 -[UIWindow _sendTouchesForEvent:] + 524
7	 UIKit	 -[UIApplication sendEvent:] + 380
8	 UIKit	 _UIApplicationHandleEvent + 6198
9	 GraphicsServices	 _PurpleEventCallback + 590
10	 GraphicsServices	 PurpleEventCallback + 34
11 ...	 CoreFoundation	 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 34
17	 UIKit	 UIApplicationMain + 1120
18	 StatusShuffleLite	
main.m line 14
main