ContextMenu.Open should call Open(null) so it doesn't break working scenarios

src/Avalonia.Controls/ContextMenu.cs

@@ -246,7 +246,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Opens the menu.
         /// </summary>
-        public override void Open() => throw new NotSupportedException();
+        public override void Open() => Open(null);
 
         /// <summary>
         /// Opens a context menu on the specified control.