[release/11.0.1xx-preview7] Avoid null TypedBinding source during ancestor detach

[PureWeen]

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Root Cause

TypedBinding.ApplyCore caches a successful TSource type check. Relative source ancestor bindings can temporarily resolve to null when an item is detached or recycled, but the cached result remained true. This caused the generated binding getter to be invoked with a null source, producing a first-chance NullReferenceException that was swallowed by the binding engine but surfaced in the debugger.

This became visible in Preview 7 after #34408 began compiling RelativeSource AncestorType bindings into TypedBinding instances.

Description of Change

Do not reuse the cached successful source type result when the current source is null. This skips subscriptions and getter invocation during the transient detach state while preserving the cached result when the ancestor is resolved again.

Add a regression test that detaches a FindAncestorBindingContext target and verifies its typed getter is never invoked with a null source.

Issues Fixed

• https://devdiv.visualstudio.com/DevDiv/_workitems/edit/3042564

Test Coverage

• RelativeSourceBindingTests

[github-actions]

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[Copilot]

Pull request overview

This PR updates the TypedBinding engine to avoid invoking typed binding getters (and related subscription logic) when a relative-source binding temporarily resolves to a null source during ancestor detach/recycle scenarios. It also adds a unit test to prevent regressions for FindAncestorBindingContext typed bindings.

Changes:

• Update TypedBinding.ApplyCore to avoid reusing a cached “is TSource” result when the current source object is null.
• Add a regression unit test ensuring a typed getter is not invoked with a null source when an ancestor binding context becomes temporarily unresolved.

Show a summary per file

File	Description
src/Controls/src/Core/TypedBinding.cs	Prevents cached typed-source checks from being applied when the current resolved source is null, avoiding getter invocation during transient detach states.
src/Controls/tests/Core.UnitTests/RelativeSourceBindingTests.cs	Adds a regression test covering FindAncestorBindingContext typed binding behavior during detach to ensure the getter isn’t called with a null source.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0