Fix crash when accessing weak reference

[scarlac]

Crash with message uiManagerWillPerformMounting: Attempted to dereference garbage pointer <address>

Summary

We are seeing multiple crashes in production apps where uiManagerWillPerformMounting is mentioned in the context of a PHPicker controller being shown. While it's not clear how to reliable reproduce the issue, after reviewing the culprit, it does seem like an issue that a weak reference hash map is being used to access shadow views. Between being added to the map and being called they could be freed, causing this crash.

Changelog

[IOS] [FIXED] - Fixed "uiManagerWillPerformMounting" native crash while another UIViewController is active.

Test Plan

Code was NOT tested. Reproducibility is an issue, and I was not able to gather any feedback on this proposal outside of a GH PR. I would like some feedback before we merge.
However, it's clear that weak references should always be checked so it seems fine to do this.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,475,498	-180
android	hermes	armeabi-v7a	7,796,965	-363
android	hermes	x86	8,951,524	-203
android	hermes	x86_64	8,809,332	-355
android	jsc	arm64-v8a	9,113,276	-186
android	jsc	armeabi-v7a	8,309,955	-356
android	jsc	x86	9,164,867	-195
android	jsc	x86_64	9,423,750	-357

Base commit: 2e3f55a
Branch: main

[javache]

Your changes will not resolve the issue at hand.

In Objective-C it's safe to call methods on nil objects, they will just become no-ops. NSHashTable will also filter out weak elements that have become deallocated when iterating.

If the pointer is a garbage value like 0xC, an if check will not filter then out.

[scarlac]

Thanks. Not sure how to fix the issue of garbage pointer then but you are probably right.