There are three possible workarounds until an official fix is available

- use RenderMode=Classic


- add a check for the object fields in the handler, in every handler

			function theCloseEventsHandler(sender, args) {
				var arg = args.get_argument();
				if (arg.target && arg.which) { //it is the mouse event
					arg = null;
				}
				alert(arg);
			}


- add the check by overriding the built-in function of the dialog. Place the following script at the end of the page that hosts the RadWindows:

			var oldClose = Telerik.Web.UI.RadWindow.prototype.close;
			Telerik.Web.UI.RadWindow.prototype.close = function (arguments) {
				if (arguments.target && arguments.which) { //it is the mouse event
					arguments = null;
				}
				var _oldClose = Function.createDelegate(this, oldClose);
				_oldClose(arguments);
			}