performBatchUpdates.crash

Crash from AppStore. Maybe you can help me to understand what happened. I use Text and Photo messages.

```
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Triggered by Thread: 0

Last Exception Backtrace:
0 CoreFoundation 0x18514ed8c __exceptionPreprocess + 228 (NSException.m:166)
1 libobjc.A.dylib 0x1843085ec objc_exception_throw + 56 (objc-exception.mm:521)
2 CoreFoundation 0x18514ebf8 +[NSException raise:format:arguments:] + 104 (NSException.m:132)
3 Foundation 0x185b3efa0 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 112 (NSException.m:157)
4 UIKit 0x18ee69db8 -[UICollectionView _endItemAnimationsWithInvalidationContext:tentativelyForReordering:animator:] + 12908 (UICollectionView.m:6031)
5 UIKit 0x18ee66b38 -[UICollectionView _endUpdatesWithInvalidationContext:tentativelyForReordering:animator:] + 92 (UICollectionView.m:6892)
6 UIKit 0x18ee65cc4 -[UICollectionView _performBatchUpdates:completion:invalidationContext:tentativelyForReordering:animator:] + 388 (UICollectionView.m:6950)
7 UIKit 0x18ee65b20 -[UICollectionView _performBatchUpdates:completion:invalidationContext:tentativelyForReordering:] + 96 (UICollectionView.m:6914)
8 UIKit 0x18ee65aa4 -[UICollectionView _performBatchUpdates:completion:invalidationContext:] + 84 (UICollectionView.m:6909)
9 UIKit 0x18ee65a3c -[UICollectionView performBatchUpdates:completion:] + 64 (UICollectionView.m:6898)
10 Chatto 0x1011c8a9c closure #3 in BaseChatViewController.performBatchUpdates(updateModelClosure:changes:updateType:completion:) + 484 (BaseChatViewController+Changes.swift:191)
11 Chatto 0x1011d3534 thunk for @escaping @callee_guaranteed () -> () + 36 (BaseChatViewController+Scrolling.swift:0)
12 UIKit 0x18ed173d4 +[UIView(UIViewAnimationWithBlocks) _setupAnimationWithDuration:delay:view:options:factory:animations:start:animationStateGenerator:completion:] + 608 (UIView.m:12443)
13 UIKit 0x18ee01090 +[UIView(UIViewAnimationWithBlocks) animateWithDuration:animations:] + 60 (UIView.m:12512)
14 Chatto 0x1011ce688 specialized BaseChatViewController.performBatchUpdates(updateModelClosure:changes:updateType:completion:) + 2932 (BaseChatViewController+Changes.swift:189)
15 Chatto 0x1011c94c4 closure #1 in BaseChatViewController.updateModels(newItems:oldItems:updateType:completion:) + 204 (BaseChatViewController+Changes.swift:0)
16 Chatto 0x1011cf460 partial apply for closure #1 in BaseChatViewController.updateModels(newItems:oldItems:updateType:completion:) + 32 (BaseChatViewController+Changes.swift:0)
17 Chatto 0x1011c975c closure #1 in closure #3 in BaseChatViewController.updateModels(newItems:oldItems:updateType:completion:) + 136 (BaseChatViewController+Changes.swift:257)
18 Chatto 0x1011cfd58 partial apply for closure #1 in closure #3 in BaseChatViewController.updateModels(newItems:oldItems:updateType:completion:) + 44 (BaseChatViewController+Changes.swift:0)
19 Chatto 0x1011d3534 thunk for @escaping @callee_guaranteed () -> () + 36 (BaseChatViewController+Scrolling.swift:0)
20 libdispatch.dylib 0x184a40b24 _dispatch_call_block_and_release + 24 (init.c:994)
21 libdispatch.dylib 0x184a40ae4 _dispatch_client_callout + 16 (object.m:507)
22 libdispatch.dylib 0x184a4d6e0 _dispatch_main_queue_callback_4CF$VARIANT$mp + 1012 (inline_internal.h:2500)
23 CoreFoundation 0x1850f7070 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12 (CFRunLoop.c:1815)
24 CoreFoundation 0x1850f4bc8 __CFRunLoopRun + 2272 (CFRunLoop.c:3111)
25 CoreFoundation 0x185014da8 CFRunLoopRunSpecific + 552 (CFRunLoop.c:3245)
26 GraphicsServices 0x186ff7020 GSEventRunModal + 100 (GSEvent.c:2245)
27 UIKit 0x18eff578c UIApplicationMain + 236 (UIApplication.m:3965)
```

Start by reviewing BaseChatViewController+Changes.swift, especially performBatchUpdates at lines 189 and 191 and updateModels around line 257. Reproduce the crash with Text and Photo messages, then determine the failing batch-update condition and verify that the same scenario no longer crashes.

Written by the indexing model from the issue text.