Ensure field is initialized. (#17218)

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

@@ -109,7 +109,7 @@ protected:
     bool _isModal;
 
 public:
-    WindowBaseImpl* Parent;
+    WindowBaseImpl* Parent = nullptr;
     NSWindow * Window;
     ComPtr<IAvnWindowBaseEvents> BaseEvents;
 };