Interpretation of iOS crash log with -[UIApplication _cachedSystemAnimationFenceCreatingIfNecessary:] - ios

Question

My submission to the App Store was rejected due to a crash. However, I am unable to reproduce the crash on multiple hardware devices and simulators. I've symbolicated the crash log, and it seems the issue is occurring in an area of code where the app is fetching some data from a server and closing a UIAlertController that notifies the user to "Please wait..." Below is the crash log.
Date/Time: 2017-05-06 00:27:06.7009 +0700
Launch Time: 2017-05-06 00:26:43.8793 +0700
OS Version: iPhone OS 10.3.1 (14E304)
Report Version: 104
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Triggered by Thread: 1
Application Specific Information:
abort() called
Filtered syslog:
None found
Last Exception Backtrace:
0 CoreFoundation 0x18c47efd8 __exceptionPreprocess + 124
1 libobjc.A.dylib 0x18aee0538 objc_exception_throw + 56
2 CoreFoundation 0x18c47eeac +[NSException raise:format:arguments:] + 104
3 Foundation 0x18cf16710 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 112
4 UIKit 0x192823454 -[UIApplication _cachedSystemAnimationFenceCreatingIfNecessary:] + 196
5 UIKit 0x192823514 -[UIApplication _systemAnimationFenceCreatingIfNecessary:] + 24
6 UIKit 0x19287c7a0 +[UIWindow _synchronizedDrawingFence] + 116
7 UIKit 0x192a3a150 +[_UIKeyboardChangedInformation informationForKeyboardUp:withIAV:] + 104
8 UIKit 0x192d338d4 -[_UIRemoteKeyboards prepareToMoveKeyboard:withIAV:showing:forScreen:] + 464
9 UIKit 0x19266bc04 -[UIPeripheralHost(UIKitInternal) setInputViews:animationStyle:] + 820
10 UIKit 0x192688cf4 -[UIPeripheralHost(UIKitInternal) _restoreInputViewsWithId:animated:] + 584
11 UIKit 0x19291bd64 -[UIViewController _restoreInputViewsForPresentation] + 280
12 UIKit 0x1928f6e3c -[UIPresentationController runTransitionForCurrentState] + 1560
13 UIKit 0x1928f50f8 -[UIPresentationController _dismissWithAnimationController:interactionController:target:didEndSelector:] + 764
14 UIKit 0x19291b8c0 -[UIViewController _dismissViewControllerWithAnimationController:interactionController:completion:] + 464
15 UIKit 0x1926d0cd4 -[UIViewController _dismissViewControllerWithTransition:from:completion:] + 600
16 UIKit 0x192678b6c -[UIViewController dismissViewControllerWithTransition:completion:] + 1144
17 UIKit 0x19267894c -[UIViewController dismissViewControllerWithTransition:completion:] + 600
18 UIKit 0x19291ab68 -[UIViewController _performCoordinatedPresentOrDismiss:animated:] + 524
19 UIKit 0x1926786c0 -[UIViewController dismissViewControllerAnimated:completion:] + 152
20 DinokEtp 0x1001014c4 specialized ViewController.(downloadCaseTypesFile(workFlowVersionMobileShouleBeOn : Double, validateUserResponse : ValidateUserResponse) -> ()).(closure #1) (ViewController.swift:360)
21 DinokEtp 0x1000fd84c ViewController.(downloadCaseTypesFile(workFlowVersionMobileShouleBeOn : Double, validateUserResponse : ValidateUserResponse) -> ()).(closure #1) (ViewController.swift:0)
22 DinokEtp 0x100101b38 partial apply for ViewController.(downloadCaseTypesFile(workFlowVersionMobileShouleBeOn : Double, validateUserResponse : ValidateUserResponse) -> ()).(closure #1) (ViewController.swift:0)
23 CFNetwork 0x18ca89358 __85-[__NSURLSessionLocal downloadTaskForRequest:downloadFilePath:resumeData:completion:]_block_invoke + 32
24 CFNetwork 0x18caeb18c __50-[__NSCFLocalDownloadTask _private_fileCompletion]_block_invoke.321 + 84
25 Foundation 0x18cf4d804 __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 16
26 Foundation 0x18ce92760 -[NSBlockOperation main] + 96
27 Foundation 0x18ce82b18 -[__NSOperationInternal _start:] + 612
28 Foundation 0x18cf4fba0 __NSOQSchedule_f + 228
29 libdispatch.dylib 0x18b3369a0 _dispatch_client_callout + 16
30 libdispatch.dylib 0x18b344ad4 _dispatch_queue_serial_drain + 928
31 libdispatch.dylib 0x18b33a2cc _dispatch_queue_invoke + 884
32 libdispatch.dylib 0x18b346a50 _dispatch_root_queue_drain + 540
33 libdispatch.dylib 0x18b3467d0 _dispatch_worker_thread3 + 124
34 libsystem_pthread.dylib 0x18b53f1d0 _pthread_wqthread + 1096
35 libsystem_pthread.dylib 0x18b53ed7c start_wqthread + 4
Thread 0 name: Dispatch queue: com.apple.main-thread
Thread 0:
0 libsystem_kernel.dylib 0x000000018b45b224 mach_msg_trap + 8
1 libsystem_kernel.dylib 0x000000018b45b09c mach_msg + 72
2 CoreFoundation 0x000000018c42ce88 __CFRunLoopServiceMachPort + 192
3 CoreFoundation 0x000000018c42aadc __CFRunLoopRun + 1060
4 CoreFoundation 0x000000018c35ad94 CFRunLoopRunSpecific + 424
5 GraphicsServices 0x000000018ddc4074 GSEventRunModal + 100
6 UIKit 0x000000019260c130 UIApplicationMain + 208
7 DinokEtp 0x00000001000654bc main (EMSCaseUpload.swift:14)
8 libdyld.dylib 0x000000018b36959c start + 4
Thread 1 name: Dispatch queue: NSOperationQueue 0x17003e2c0 :: NSOperation 0x17025e5d0 (QOS: DEFAULT)
Thread 1 Crashed:
0 libsystem_kernel.dylib 0x000000018b479014 __pthread_kill + 8
1 libsystem_pthread.dylib 0x000000018b543334 pthread_kill + 112
2 libsystem_c.dylib 0x000000018b3ed9c4 abort + 140
3 libc++abi.dylib 0x000000018aeb91b0 __cxa_bad_cast + 0
4 libc++abi.dylib 0x000000018aed2c04 default_unexpected_handler() + 0
5 libobjc.A.dylib 0x000000018aee0820 _objc_terminate() + 124
6 libc++abi.dylib 0x000000018aecf5d4 std::__terminate(void (*)()) + 16
7 libc++abi.dylib 0x000000018aecf640 std::terminate() + 60
8 libdispatch.dylib 0x000000018b3369b4 _dispatch_client_callout + 36
9 libdispatch.dylib 0x000000018b344ad4 _dispatch_queue_serial_drain + 928
10 libdispatch.dylib 0x000000018b33a2cc _dispatch_queue_invoke + 884
11 libdispatch.dylib 0x000000018b346a50 _dispatch_root_queue_drain + 540
12 libdispatch.dylib 0x000000018b3467d0 _dispatch_worker_thread3 + 124
13 libsystem_pthread.dylib 0x000000018b53f1d0 _pthread_wqthread + 1096
14 libsystem_pthread.dylib 0x000000018b53ed7c start_wqthread + 4
Thread 2:
0 libsystem_pthread.dylib 0x000000018b53ed78 start_wqthread + 0
Thread 3 name: com.apple.uikit.eventfetch-thread
Thread 3:
0 libsystem_kernel.dylib 0x000000018b45b224 mach_msg_trap + 8
1 libsystem_kernel.dylib 0x000000018b45b09c mach_msg + 72
2 CoreFoundation 0x000000018c42ce88 __CFRunLoopServiceMachPort + 192
3 CoreFoundation 0x000000018c42aadc __CFRunLoopRun + 1060
4 CoreFoundation 0x000000018c35ad94 CFRunLoopRunSpecific + 424
5 Foundation 0x000000018ce74d64 -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 304
6 Foundation 0x000000018ce95b34 -[NSRunLoop(NSRunLoop) runUntilDate:] + 96
7 UIKit 0x0000000192f965f8 -[UIEventFetcher threadMain] + 136
8 Foundation 0x000000018cf722c8 __NSThread__start__ + 996
9 libsystem_pthread.dylib 0x000000018b54175c _pthread_body + 240
10 libsystem_pthread.dylib 0x000000018b54166c _pthread_body + 0
11 libsystem_pthread.dylib 0x000000018b53ed84 thread_start + 4
Thread 4:
0 libsystem_kernel.dylib 0x000000018b479a88 __workq_kernreturn + 8
1 libsystem_pthread.dylib 0x000000018b53f274 _pthread_wqthread + 1260
2 libsystem_pthread.dylib 0x000000018b53ed7c start_wqthread + 4
Thread 5:
0 libsystem_pthread.dylib 0x000000018b53ed78 start_wqthread + 0
Thread 6 name: com.apple.NSURLConnectionLoader
Thread 6:
0 libsystem_kernel.dylib 0x000000018b45b224 mach_msg_trap + 8
1 libsystem_kernel.dylib 0x000000018b45b09c mach_msg + 72
2 CoreFoundation 0x000000018c42ce88 __CFRunLoopServiceMachPort + 192
3 CoreFoundation 0x000000018c42aadc __CFRunLoopRun + 1060
4 CoreFoundation 0x000000018c35ad94 CFRunLoopRunSpecific + 424
5 CFNetwork 0x000000018cb66ca4 +[NSURLConnection(Loader) _resourceLoadLoop:] + 404
6 Foundation 0x000000018cf722c8 __NSThread__start__ + 996
7 libsystem_pthread.dylib 0x000000018b54175c _pthread_body + 240
8 libsystem_pthread.dylib 0x000000018b54166c _pthread_body + 0
9 libsystem_pthread.dylib 0x000000018b53ed84 thread_start + 4
Thread 1 crashed with ARM Thread State (64-bit):
x0: 0x0000000000000000 x1: 0x0000000000000000 x2: 0x0000000000000000 x3: 0x00000001700f8c37
x4: 0x000000018aed3b07 x5: 0x000000016de9a850 x6: 0x000000000000006e x7: 0xffffffffffffffec
x8: 0x000000000c000000 x9: 0x0000000004000000 x10: 0x000000000000000b x11: 0x000000000000000b
x12: 0x0000000000000010 x13: 0x000000018c6c9c0e x14: 0x0000030000000300 x15: 0x0000000000000000
x16: 0x0000000000000148 x17: 0x0000000000000000 x18: 0x0000000000000000 x19: 0x0000000000000006
x20: 0x000000016de9b000 x21: 0x000000016de9a850 x22: 0x00000001700ff380 x23: 0x0000000000000004
x24: 0x00000001b1ebd400 x25: 0x0000000000000000 x26: 0x00000001b1ebd400 x27: 0x000000016de9b0e0
x28: 0x0000000000000000 fp: 0x000000016de9a7b0 lr: 0x000000018b543334
sp: 0x000000016de9a790 pc: 0x000000018b479014 cpsr: 0x00000000

Answer 1

I would recommend double checking that you are not interacting with that UIAlertController (or any other UIKit API) outside the main thread (aka, Thread 0 above). Many UIKit APIs are designed to be used only from the main thread.
Worth noting that the crash happened on Thread 1…
If you are in doubt, wrap the call with:
DispatchQueue.main.async {
// Call UIAlertController code here...
}