Added WindowBase.TryGetPlatformHandle.

#11062 made `WindowBase.PlatformImpl` internal so we need to expose a different want to get the window handle.

src/Avalonia.Controls/WindowBase.cs

@@ -160,6 +160,15 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Trys to get the platform handle for the window.
+        /// </summary>
+        /// <returns>
+        /// An <see cref="IPlatformHandle"/> describing the window handle, or null if the handle
+        /// could not be retrieved.
+        /// </returns>
+        public IPlatformHandle? TryGetPlatformHandle() => PlatformImpl?.Handle;
+
         /// <summary>
         /// Ensures that the window is initialized.
         /// </summary>