[iOS] Fix for NullReferenceException in WillMoveToParentViewController When Removing Page During Navigation

[praveenkumarkarunanithi]

Root Cause

The NullReferenceException occurs when a non-current page is removed from the navigation stack. Specifically, when RemovePage is invoked for a page that is not the current one, the NavigationPage.Legacy.RemovePage method synchronously calls _removePageRequested?.Invoke(this, new NavigationRequestedEventArgs(page, true)). This invocation triggers the RemovePage method in the NavigationRenderer, which immediately calls Dispose(). The Dispose() method then calls Disconnect(), setting the Tracker to null before the native WillMoveToParentViewController method is executed. As a result, when WillMoveToParentViewController attempts to access the now null Tracker, a NullReferenceException is thrown.

In contrast, removing the current page using PopAsync handles the removal asynchronously. This ensures that WillMoveToParentViewController is called while the Tracker remains valid. Only after the native operations complete does Disconnect(false) nullify the Tracker. This proper sequencing maintains the integrity of the Tracker during native method execution, preventing the exception when using PopAsync.

Description of Change

The fix adds a null check for ToolbarTracker in the WillMoveToParentViewController method, ensuring it is accessed only when initialized, thereby preventing the NullReferenceException.

Issues Fixed

Fixes #25726
Tested the behaviour in the following platforms

• Android
• Windows
• iOS
• Mac

Screenshots

BeforeFix.mov

AfterFix.mov

[dotnet-policy-service]

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

[jsuarezruiz]

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