在viewdidappear上，我提出了一个alertcontroller并调用异步数据库请求 一旦收到数据，我将在主要部分中解除警报 自动线程，无需任何用户交互。
此时，我的应用程序崩溃但无法重现。
这是随机发生的崩溃，非常罕见地重现。
这是崩溃报告
> Crashed: com.apple.main-thread 0 UIKit
> 0x255dd374 -[UIPresentationController transitionDidFinish:] + 95 1
> UIKit 0x255dd371
> -[UIPresentationController transitionDidFinish:] + 92 2 UIKit 0x255e0a85 __56-[UIPresentationController
> runTransitionForCurrentState]_block_invoke_2 + 192 3 UIKit
> 0x253be157 -[_UIViewControllerTransitionContext completeTransition:] +
> 90 4 UIKit 0x252cbba5
> -[UIViewAnimationBlockDelegate _didEndBlockAnimation:finished:context:] + 540 5 UIKit 0x252cb685 -[UIViewAnimationState
> sendDelegateAnimationDidStop:finished:] + 204 6 UIKit
> 0x252cb57f -[UIViewAnimationState animationDidStop:finished:] + 78 7
> QuartzCore 0x2332c699
> CA::Layer::run_animation_callbacks(void*) + 252 8 libdispatch.dylib
> 0x2088580f _dispatch_client_callout + 22 9 libdispatch.dylib
> 0x20893ba9 _dispatch_main_queue_callback_4CF$VARIANT$mp + 1524 10
> CoreFoundation 0x20cd9b6d
> __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 8 11 CoreFoundation 0x20cd8067 __CFRunLoopRun + 1574
> 12 CoreFoundation 0x20c27229 CFRunLoopRunSpecific
> + 520 13 CoreFoundation 0x20c27015 CFRunLoopRunInMode + 108 14 GraphicsServices
> 0x22217ac9 GSEventRunModal + 160 15 UIKit
> 0x252f9189 UIApplicationMain + 144 16 MY_Test_App 0x1b6397 main
> (main.m:16) 17 libdyld.dylib 0x208cf873 start +
> 2