Fixes crash when accessing a cell within working range updates

[rnystrom]

Changes in this pull request

I was able to build a unit test that reproduces the issue. We can avoid the crash by simply returning nil when accessing a cell while working range events are being vended.

There is definitely something weird going on here though. When debugging cellForItemAtIndexPath: I found:

(lldb) po indexPath
<NSIndexPath: 0xc000000000000516> {length = 2, path = 5 - 0}
(lldb) po [[self collectionView] numberOfSections]
11

So in theory we should be fine, right? But when I continue I get

*** Assertion failure in -[UICollectionViewData numberOfItemsBeforeSection:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit_Sim/UIKit-3600.5.2/UICollectionViewData.m:611
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'request for number of items before section 5 when there are only 3 sections in the collection view'

There were 3 sections in the UICV before the update, but the data source and structure powering numberOfSections are updated.

-numberOfItemsBeforeSection: must be accessing some state that isn't set yet from the batch updates block. Unfortunately the details of that are private. Ideally -[UICollectionView cellForItemAtIndexPath:] would just return nil if it hasn't finished updating yet.

I'm a little skeptical about this fix, and this might require filing a radar with Apple.

Fixes #177

Pull request checklist

• All tests pass. Demo project builds and runs.
• I added tests, an experiment, or detailed why my change isn't tested.
• I have reviewed the contributing guide

[jessesquires]

Nice! 👍

Update CHANGELOG 😄

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