[iOS] Fixed a crash issue with CollectionView when configured with CollectionViewHandler2 on iOS 15 and iOS 16.

[Tamilarasan-Paranthaman]

Root Cause of the issue

• On iOS versions 15 and 16, the NumberOfSections method is called immediately after the base.LoadView method is invoked. At this point, ItemsSource was not set, which caused a crash when accessing the details from ItemsSource.

Description of Change

• To address this issue, I added a check to verify if ItemsSource is null before proceeding. On iOS 17 and later, this behavior works correctly as the method is no longer called prematurely. For compatibility with older iOS versions, the null check ensures the method returns 0 if ItemsSource is null, resolving the issue with CV2 on iOS 15 and 16

Issues Fixed

Fixes #26997

Tested the behaviour in the following platforms

• iOS
• Android
• Windows
• Mac

Tested Versions

iOS Version	Did the App crash?
iOS 15.2	✅
iOS 15.5	✅
iOS 16.4	✅
iOS 17.5	❌
iOS 18.0	❌
iOS 18.1	❌

Screenshots

Note: The video attached below shows that this issue occurs only on iOS 15+ and 16+ versions.

iOS version	Before Issue Fix	After Issue Fix
iOS 15.2	Before-Fix.mov	After-Fix.mov
iOS 15.5	Before-15.5.mov	After-15.5.mov
iOS 16.4	Before-16.4.mov	After-16.4.mov
iOS 17.5	Before-17.5.mov	After-17.5.mov
iOS 18.1	Before-18.1.mov	After-18.1.mov

[dotnet-policy-service]

Hey there @Tamilarasan-Paranthaman! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jfversluis]

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