[MacOS] fix small memory leak. base.Dispose so that _nativeControlHost and _mouse are disposed correctly.

src/Avalonia.Native/WindowImplBase.cs

@@ -78,8 +78,7 @@ namespace Avalonia.Native
         public override void Dispose()
         {
             Native?.Close();
-            Native?.Dispose();
+            base.Dispose();
-            _handle = null;
         }
 
         public virtual void Show(bool activate, bool isDialog)