Avoid NRE in DefaultMenuInteractionHandler.

The input can arrive after the interaction handler has already been detached.

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -377,7 +377,7 @@ namespace Avalonia.Controls.Platform
 
             if (mouse?.Type == RawPointerEventType.NonClientLeftButtonDown)
             {
-                Menu.Close();
+                Menu?.Close();
             }
         }