Objective-C iOS app crashes during CFStringAppend for unknown reason - ios

Question

In my iPhone app, for some users the app will always crash, whenever they open a certain window. The same window works fine for me and other people, but always causes a crash for some people. It is unclear at this moment what the difference is between people for whom it fails, and for whom it works fine.
The animation to open the specific view will run fine, but will then cause a crash immediately after the animation is finished.
They have sent me the crashlog, but it is hard to understand what is exactly going wrong. I am hoping you can help me understand. The cause for the failure as given by the crashlog is the following exception:
Last Exception Backtrace:
0 CoreFoundation 0x1836ffd38 __exceptionPreprocess + 124
1 libobjc.A.dylib 0x182c14528 objc_exception_throw + 55
2 CoreFoundation 0x18370d1f8 -[NSObject+ 1372664 (NSObject) doesNotRecognizeSelector:] + 139
3 UIKit 0x18cec7cc4 -[UIResponder doesNotRecognizeSelector:] + 295
4 CoreFoundation 0x1837056e4 ___forwarding___ + 1379
5 CoreFoundation 0x1835eb0dc _CF_forwarding_prep_0 + 91
6 CoreFoundation 0x1835d6be8 CFStringAppend + 519
7 CoreFoundation 0x1836bddf0 __CFStringAppendFormatCore + 9271
8 CoreFoundation 0x1836bf658 _CFStringCreateWithFormatAndArgumentsAux2 + 131
9 AccessibilityUtilities 0x192d6b388 _AXStringForArgs + 279
10 UIKit 0x1a242adf8 -[UIViewControllerAccessibility viewDidAppear:] + 267
11 UIKit 0x18cb2869c -[UIViewController _setViewAppearState:isAnimating:] + 851
12 UIKit 0x18cb28c08 -[UIViewController _endAppearanceTransition:] + 227
13 UIKit 0x18cbcee00 -[UINavigationController navigationTransitionView:didEndTransition:fromView:toView:] + 1327
14 UIKit 0x1a2440cd4 -[UINavigationControllerAccessibility navigationTransitionView:didEndTransition:fromView:toView:] + 111
15 UIKit 0x18cc96bbc __49-[UINavigationController _startCustomTransition:]_block_invoke + 251
16 UIKit 0x18cc229d8 -[_UIViewControllerTransitionContext completeTransition:] + 115
17 UIKit 0x18cd67d30 __53-[_UINavigationParallaxTransition animateTransition:]_block_invoke.124 + 751
18 UIKit 0x18cb47d7c -[UIViewAnimationBlockDelegate _didEndBlockAnimation:finished:context:] + 763
19 UIKit 0x18cb4770c -[UIViewAnimationState sendDelegateAnimationDidStop:finished:] + 311
20 UIKit 0x18cb47418 -[UIViewAnimationState animationDidStop:finished:] + 295
21 UIKit 0x1a2468970 -[UIViewAnimationStateAccessibility animationDidStop:finished:] + 131
22 QuartzCore 0x1876ebd6c CA::Layer::run_animation_callbacks+ 1232236 (void*) + 283
23 libdispatch.dylib 0x183085048 _dispatch_client_callout + 15
24 libdispatch.dylib 0x183091b74 _dispatch_main_queue_callback_4CF$VARIANT$mp + 1015
25 CoreFoundation 0x1836a7f20 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 11
26 CoreFoundation 0x1836a5afc __CFRunLoopRun + 2011
27 CoreFoundation 0x1835c62d8 CFRunLoopRunSpecific + 435
28 GraphicsServices 0x185457f84 GSEventRunModal + 99
29 UIKit 0x18cb73880 UIApplicationMain + 207
30 Flyskyhy 0x10490bd80 main + 32128 (main.m:17)
31 libdyld.dylib 0x1830ea56c start + 3
The crashlog seems to indicate that it goes wrong while doing a CFStringAppend, but it is not clear which string is the problem, or what is wrong with it, or even why a CFStringAppend is needed. All strings visible in the view have been filled in already before the animation starts, and they all are correct.
EDIT:
As requested, here is the code that starts the view. Everything is under a NavigationController, so the new view controller is pushed on the navigation stack to open it.
WaypointEditController *controller = [[WaypointEditController alloc] initWithNibName:#"WayPointEdit" bundle:nil];
controller.navigationItem.title = #"New Waypoint";
// initialisation of other, custom, fields of controller
[self.navigationController pushViewController:controller animated:YES];
The WaypointEditController class that is pushed is derived from UIViewController. The viewWillAppear is overridden, to do initialisation of the fields of the view. But - relevant here - viewDidAppear is not overridden.
In case relevant, here are the most important actions in the viewWillAppear method:
- (void) viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
[self.navigationController setNavigationBarHidden:NO animated:YES];
self.navigationItem.title = #"New Waypoint";
// other initialisation of internal fields
}

Answer 1

Like you, I have customers reporting this problem and I could see the stack traces that XCode downloaded from production but I could not reproduce it... until I tried enabling "voice over".
The other accessibility options that I tried worked OK.
The stack trace shows that iOS is sending a selector of "length" to an instance of UILabel. This seemed odd. To see what would happen, I added a "length" method to UILabel to return the length of the UILabel's text property. Then it just failed with an unknown selector of _encodingCantBeStoredInEightBitCFString. So clearly iOS thought that my UILabel was something that it wasn't.
This ended up being because "description" was used for the name of my UILabel property/IBOutlet.
To fix the problem I renamed the property from "description" to something different. I suspect that my synthesized "description" getter was overriding NSObject's description method (which returns an NSString which fits the selectors which were being sent to my UILabel).