title: UICollectionViewData updateItemCounts Crash
body: <p>I'm getting crashes from deep inside Apple's layout code for a UICollectionViewFlowLayout and I have no idea how to address these (stack trace below). Any suggestions would be greatly appreciated.</p>
  
  <p>Details:</p>
  
  <ul>
  <li>Unfortunately I can't reproduce the issue</li>
  <li>The issue isn't frequent (it happens in less than 0.1% of sessions) but it's our #1 crash</li>
  </ul>
  
  <pre class="lang-none prettyprint-override"><code>#0. Crashed: com.apple.main-thread
  0  libsystem_platform.dylib       0x1879490d4 __bzero + 36
  1  UIKit                          0x18e824e40 -[UICollectionViewData _updateItemCounts] + 544
  2  UIKit                          0x18e824e40 -[UICollectionViewData _updateItemCounts] + 544
  3  UIKit                          0x18e8e67e0 -[UICollectionViewData numberOfSections] + 28
  4  UIKit                          0x18f086bc0 -[UICollectionViewFlowLayout _getSizingInfosWithExistingSizingDictionary:] + 612
  5  UIKit                          0x18f088834 -[UICollectionViewFlowLayout _fetchItemsInfoForRect:] + 152
  6  UIKit                          0x18e8e66b4 -[UICollectionViewFlowLayout prepareLayout] + 224
  7  UIKit                          0x18e7cf574 -[UICollectionViewData _prepareToLoadData] + 164
  8  UIKit                          0x18e7ceb5c -[UICollectionViewData validateLayoutInRect:] + 100
  9  UIKit                          0x18e7ce55c -[UICollectionView layoutSubviews] + 212
  10 UIKit                          0x18e76fa80 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1196
  11 QuartzCore                     0x18bc1d9d8 -[CALayer layoutSublayers] + 148
  12 QuartzCore                     0x18bc124cc CA::Layer::layout_if_needed(CA::Transaction*) + 292
  13 QuartzCore                     0x18bc1238c CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 32
  14 QuartzCore                     0x18bb8f3e0 CA::Context::commit_transaction(CA::Transaction*) + 252
  15 QuartzCore                     0x18bbb6a68 CA::Transaction::commit() + 512
  16 QuartzCore                     0x18bbb7488 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 120
  17 CoreFoundation                 0x18886a0c0 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 32
  18 CoreFoundation                 0x188867cf0 __CFRunLoopDoObservers + 372
  19 CoreFoundation                 0x188868180 __CFRunLoopRun + 1024
  20 CoreFoundation                 0x1887962b8 CFRunLoopRunSpecific + 444
  21 GraphicsServices               0x18a24a198 GSEventRunModal + 180
  22 UIKit                          0x18e7dd7fc -[UIApplication _run] + 684
  23 UIKit                          0x18e7d8534 UIApplicationMain + 208
  24 AppName Mobile                 0x1000333e8 main (main.m:16)
  25 libdispatch.dylib              0x1877795b8 (Missing)
  </code></pre>