Fix TabbedPage crash on iPadOS 26+ with narrow viewports

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Description of Change

On iPadOS 26.1+, TabbedPage crashes with NullReferenceException in windowed apps < 667pt width. iOS 26.1 introduced early ViewDidLayoutSubviews() calls during UITabBarController base constructor execution for narrow viewports, before _viewHandlerWrapper initialization completes.

Fix: Add null-conditional operator to Element property getter:

- public VisualElement Element => _viewHandlerWrapper.Element ?? _element?.GetTargetOrDefault();
+ public VisualElement Element => _viewHandlerWrapper?.Element ?? _element?.GetTargetOrDefault();

The existing if (Element is IView view) guard in ViewDidLayoutSubviews() handles null safely.

Issues Fixed

Fixes #32661

Original prompt

Backport #32662

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Pull Request Overview

This PR fixes a NullReferenceException crash in TabbedPage on iPadOS 26.1+ when running in narrow viewports (< 667pt width). The crash occurs because iOS 26.1 introduced early ViewDidLayoutSubviews() calls during UITabBarController base constructor execution, before the _viewHandlerWrapper field is fully initialized.

Key Changes

• Added null-conditional operator (?.) to the Element property getter to prevent dereferencing a null _viewHandlerWrapper during early lifecycle events
• Added explanatory comment documenting the iPadOS 26.1+ behavior that necessitates this defensive code