Fix off-thread accessibility hit-test crash on macOS (#21779)

* Fix off-thread accessibility hit-test crash on macOS

GetAutomationPeer() returns null when called off the UI thread, but
-[AvnWindow automationPeer] called SetNode on it without checking, which
segfaults. Guard against the null peer so it returns nil instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Removed comment

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

native/Avalonia.Native/src/OSX/AvnWindow.mm

@@ -628,14 +628,18 @@
     return GetNSStringAndRelease(automationPeer->GetAutomationId());
 }
 
-- (IAvnAutomationPeer* _Nonnull) automationPeer
+- (IAvnAutomationPeer* _Nullable) automationPeer
 {
     auto parent = _parent.tryGet();
     if (parent && _automationPeer == nullptr)
     {
-        _automationPeer = parent->BaseEvents->GetAutomationPeer();
-        _automationNode = new AvnAutomationNode(self);
-        _automationPeer->SetNode(_automationNode);
+        auto peer = parent->BaseEvents->GetAutomationPeer();
+        if (peer != nullptr)
+        {
+            _automationPeer = peer;
+            _automationNode = new AvnAutomationNode(self);
+            _automationPeer->SetNode(_automationNode);
+        }
     }
 
     return _automationPeer;

native/Avalonia.Native/src/OSX/WindowProtocol.h

@@ -17,7 +17,7 @@ struct IAvnAutomationPeer;
 -(void) showAppMenuOnly;
 -(void) showWindowMenuWithAppMenu;
 -(void) applyMenu:(AvnMenu* _Nullable)menu;
--(IAvnAutomationPeer* _Nonnull) automationPeer;
+-(IAvnAutomationPeer* _Nullable) automationPeer;
 
 -(double) getExtendedTitleBarHeight;
 -(void) setIsExtended:(bool)value;