iOS 11. What the KVO_IS_RETAINING_ALL_OBSERVERS_OF_THIS_OBJECT_IF_IT_CRASHES_AN_OBSERVER_WAS_OVERRELEASED_OR_SMASHED is mean? : <p>In the new iOS11, I get some strange exceptions. I do not understand why this is happening. In the previous iOS, there was no such exception. Log attached:</p>

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<pre class="snippet-code-html lang-html prettyprint-override"><code>Crashed: com.apple.main-thread
0  libobjc.A.dylib                0x180a5e7e8 object_isClass + 16
1  Foundation                     0x181f013e8 KVO_IS_RETAINING_ALL_OBSERVERS_OF_THIS_OBJECT_IF_IT_CRASHES_AN_OBSERVER_WAS_OVERRELEASED_OR_SMASHED + 68
2  Foundation                     0x181eff8ec NSKeyValueWillChangeWithPerThreadPendingNotifications + 300
3  QuartzCore                     0x18555a6dc CAAnimation_setter(CAAnimation*, unsigned int, _CAValueType, void const*) + 156
4  QuartzCore                     0x18555d388 -[CAPropertyAnimation setKeyPath:] + 32
5  UIKit                          0x18a9b1a08 -[UIImageView startAnimating] + 876
6  UIKit                          0x18a9b0e78 -[UIActivityIndicatorView startAnimating] + 48
7  UIKit                          0x18a9b0174 -[UIActivityIndicatorView _didMoveFromWindow:toWindow:] + 212
8  UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
9  UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
10 UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
11 UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
12 UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
13 UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
14 UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
15 UIKit                          0x18a95845c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 712
16 UIKit                          0x18a957918 __45-[UIView(Hierarchy) _postMovedFromSuperview:]_block_invoke + 156
17 Foundation                     0x181e7c59c -[NSISEngine withBehaviors:performModifications:] + 168
18 UIKit                          0x18a95778c -[UIView(Hierarchy) _postMovedFromSuperview:] + 824
19 UIKit                          0x18a96339c -[UIView(Internal) _addSubview:positioned:relativeTo:] + 1728
20 UIKit                          0x18abb3158 __53-[_UINavigationParallaxTransition animateTransition:]_block_invoke_2 + 1660
21 UIKit                          0x18a969a84 +[UIView(Animation) performWithoutAnimation:] + 104
22 UIKit                          0x18ab23864 __53-[_UINavigationParallaxTransition animateTransition:]_block_invoke + 264
23 UIKit                          0x18ac418a4 +[UIView(Internal) _performBlockDelayingTriggeringResponderEvents:] + 220
24 UIKit                          0x18ab2321c -[_UINavigationParallaxTransition animateTransition:] + 1112
25 UIKit                          0x18aae1720 -[UINavigationController _startCustomTransition:] + 3444
26 UIKit                          0x18aa02e04 -[UINavigationController _startDeferredTransitionIfNeeded:] + 712
27 UIKit                          0x18aa02a34 -[UINavigationController __viewWillLayoutSubviews] + 124
28 UIKit                          0x18aa0295c -[UILayoutContainerView layoutSubviews] + 188
29 UIKit                          0x18a959000 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1256
30 QuartzCore                     0x1855290b4 -[CALayer layoutSublayers] + 184
31 QuartzCore                     0x18552d194 CA::Layer::layout_if_needed(CA::Transaction*) + 332
32 QuartzCore                     0x18549bf24 CA::Context::commit_transaction(CA::Transaction*) + 336
33 QuartzCore                     0x1854c2340 CA::Transaction::commit() + 540
34 QuartzCore                     0x1854c3180 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 92
35 CoreFoundation                 0x1814f38b8 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 32
36 CoreFoundation                 0x1814f1270 __CFRunLoopDoObservers + 412
37 CoreFoundation                 0x1814f182c __CFRunLoopRun + 1292
38 CoreFoundation                 0x1814122d8 CFRunLoopRunSpecific + 436
39 GraphicsServices               0x1832a3f84 GSEventRunModal + 100
40 UIKit                          0x18a9bf880 UIApplicationMain + 208</code></pre>
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<p>Who ever encountered this? What is it and how to defeat it?</p>
