在生产中间歇性地获取此错误。 isjindOfClass的objMsgSend失败。难以在开发中复制，希望能够这样做。尝试在屏幕上显示和解除UIAlertController与其他视图控制器，移动到后台等等。编辑：现在已经能够重现这一点。当UIAlertController与应用程序中的现有弹出窗口一起显示（显示在下方）时发生。 EDIT2：似乎是iOS8上的一个问题，但不太常见，尽管仍然出现在iOS9中
线程：崩溃：com.apple.main-thread 0 libobjc.A.dylib
0x0000000194d8bbd0 objc_msgSend + 16 1 UIKit
0x0000000188a2d384 - [UIWindow setDelegate：] + 132 2 UIKit
0x0000000188cb5cd4 - [UIPresentationController runTransitionForCurrentState] + 592 3 UIKit
0x0000000188cd1cd8 - [UIViewController _dismissViewControllerWithAnimationController：interactionController：完成：] + 672 4 UIKit 0x0000000188ae0258 - [UIViewController _dismissViewControllerWithTransition：from：completion：] + 852 5 UIKit 0x0000000188a8875c - [UIViewController dismissViewControllerWithTransition：completion：] + 952 6 UIKit
0x0000000188a88344 - [UIViewController dismissViewControllerAnimated：completion：] + 252 7 UIKit
0x0000000188d84d7c - [UIAlertController _dismissAnimated：triggeringAction：triggeredByPopoverDimmingView：] + 420 8 UIKit 0x00000001889ac750 _UIGestureRecognizerUpdate + 8536 9 UIKit 0x00000001889e68dc - [UIWindow _sendGesturesForEvent：] + 1044 10 UIKit 0x00000001889e5f7c - [UIWindow sendEvent：] + 660 11 UIKit
0x00000001889b968c - [UIApplication sendEvent：] + 264 12 iPath
0x000000010023c3cc - [SFApplication sendEvent：]（SFApplication.m：72）13 UIKit 0x0000000188c5860c _UIApplicationHandleEventFromQueueEvent + 14992 14 UIKit 0x00000001889b7bf4 _UIApplicationHandleEventQueue + 1616 15 CoreFoundation 0x00000001841c29ec CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION + 24 16 CoreFoundation 0x00000001841c1c90 __CFRunLoopDoSources0 + 264 17 CoreFoundation 0x00000001841bfd40 __CFRunLoopRun + 712 18 CoreFoundation
0x00000001840ed0a4 CFRunLoopRunSpecific + 396 19 GraphicsServices
0x000000018d25b5a4 GSEventRunModal + 168 20 UIKit
0x0000000188a1eaa4 UIApplicationMain + 1488 21 iPath
0x00000001000ca308 main（main.m:33）22 libdyld.dylib
0x00000001953e6a08 start + 4