EXC_BAD_ACCESS when swizzle popViewControllerAnimated:

Question

For logging navigation between controllers, I use swizzling of -popViewControllersAnimated: method. And sometimes it causes strange crash with EXC_BAD_ACCESS. What can be the reason?

- (void)sw_popViewControllerAnimated:(BOOL)animated {

//1. get top controller before pop
//2. pop
//3. get top controller after pop
//4. log it
NSString *controllerFromIdentifier = @"";
__block NSString *controllerToIdentifier = @"";

    NSDictionary *controllerFromAdditionalParams = @{};

    UIViewController *topController = ((UINavigationController*)self).topViewController;

    [self sw_popViewControllerAnimated:animated];

    UIViewController *topControllerAfterPop = ((UINavigationController*)self).topViewController;

}