title: iOS app error - Can't add self as subview
body: <p>I received this crash report, but I don't know how to debug it.</p>
  
  <pre><code>Fatal Exception NSInvalidArgumentException
  Can't add self as subview
  0 ...    CoreFoundation  __exceptionPreprocess + 130
  1    libobjc.A.dylib     objc_exception_throw + 38
  2    CoreFoundation  -[NSException initWithCoder:]
  3    UIKit   -[UIView(Internal) _addSubview:positioned:relativeTo:] + 110
  4    UIKit   -[UIView(Hierarchy) addSubview:] + 30
  5    UIKit   __53-[_UINavigationParallaxTransition animateTransition:]_block_invoke + 1196
  6    UIKit   +[UIView(Animation) performWithoutAnimation:] + 72
  7    UIKit   -[_UINavigationParallaxTransition animateTransition:] + 732
  8    UIKit   -[UINavigationController _startCustomTransition:] + 2616
  9    UIKit   -[UINavigationController _startDeferredTransitionIfNeeded:] + 418
  10   UIKit   -[UINavigationController __viewWillLayoutSubviews] + 44
  11   UIKit   -[UILayoutContainerView layoutSubviews] + 184
  12   UIKit   -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 346
  13   QuartzCore  -[CALayer layoutSublayers] + 142
  14   QuartzCore  CA::Layer::layout_if_needed(CA::Transaction*) + 350
  15   QuartzCore  CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 16
  16   QuartzCore  CA::Context::commit_transaction(CA::Transaction*) + 228
  17   QuartzCore  CA::Transaction::commit() + 314
  18   QuartzCore  CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 56
  </code></pre>
  
  <p>The iOS version is 7.0.3.
  Anyone experience this weird crash?</p>
  
  <p>UPDATE:</p>
  
  <p>I don't know where in my code caused this crash, so I can not post the code here, sorry.</p>
  
  <p><strong>Second UPDATE</strong></p>
  
  <p>See the answer below.</p>