title: iOS app crashing with cfrunloop_is_calling_out_to_an_observer_callback_function
body: <p>When running my app I occasionally get this crash that seems not to have anything with my code. What is it and how my I avert it?</p>
  
  <blockquote>
    <p>Thread 0 name:  Dispatch queue: com.apple.main-thread Thread 0
    Crashed: 0   libsystem_kernel.dylib           0x000000018fc4c16c
    mach_msg_trap + 8 1   libsystem_kernel.dylib<br>
      0x000000018fc4bfdc mach_msg + 72 2   AudioToolbox<br>
      0x0000000193c8bcdc ASClient_AudioSessionSetActiveWithFlags + 132 3<br>
    AudioToolbox                      0x0000000193c6c7f4
    AudioSessionSetActive_Priv + 360 4   AVFAudio<br>
      0x00000001aa46bf8c -[AVAudioSession setActive:withOptions:error:] +
    84 5   Jam Session                    0x00000001000d41b8 0x1000b0000 +
    147896 6   Jam Session                    0x00000001000d53fc
    0x1000b0000 + 152572 7   UIKit<br>
      0x0000000196ae90ec -[UIViewController loadViewIfRequired] + 1056 8<br>
    UIKit                             0x0000000196ba2cdc
    -[UINavigationController _layoutViewController:] + 72 9   UIKit                           0x0000000196ba2bb4 -[UINavigationController
    _updateScrollViewFromViewController:toViewController:] + 416 10  UIKit                            0x0000000196ba1efc -[UINavigationController
    _startTransition:fromViewController:toViewController:] + 140 11  UIKit                            0x0000000196ba1948 -[UINavigationController
    _startDeferredTransitionIfNeeded:] + 856 12  UIKit                            0x0000000196ba14fc -[UINavigationController <strong>viewWillLayoutSubviews]
    + 64 13  UIKit                            0x0000000196ba1460 -[UILayoutContainerView layoutSubviews] + 188 14  UIKit                          0x0000000196ae625c -[UIView(CALayerDelegate) layoutSublayersOfLayer:]
    + 1196 15  QuartzCore                     0x0000000193fad2c8 -[CALayer layoutSublayers] + 148 16  QuartzCore<br>
      0x0000000193fa1fa4 CA::Layer::layout_if_needed(CA::Transaction*) +
    292 17  QuartzCore                        0x0000000193fa1e64
    CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 32 18 
    QuartzCore                        0x0000000193f1eb20
    CA::Context::commit_transaction(CA::Transaction*) + 252 19  QuartzCore
      0x0000000193f45f8c CA::Transaction::commit() + 512 20  QuartzCore<br>
      0x0000000193f469ac
    CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned
    long, void*) + 120 21  CoreFoundation<br>
      0x0000000190c4a7dc
    __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION</strong> + 32 22  CoreFoundation                 0x0000000190c4840c
    __CFRunLoopDoObservers + 372 23  CoreFoundation                   0x0000000190b77068 CFRunLoopRunSpecific + 476 24  UIKit<br>
      0x0000000196b542ec -[UIApplication _run] + 608 25  UIKit<br>
      0x0000000196b4f070 UIApplicationMain + 208 26  Jam Session<br>
      0x00000001000c36d4 0x1000b0000 + 79572 27  libdyld.dylib<br>
      0x000000018fb585b8 start + 4</p>
  </blockquote>