In IE only (not Chrome), beginning with the Q2 2013 release, an error is thrown on a RadWindow with the EnableShadow="true" property if .hide() is called on the window before it is made visible.  This is new behavior with Q2 2013.  The affected line is:

}else{Sys.UI.DomElement.removeCssClass(this.container,"rwShadow");

where .container = nothing

Current workarounds are:

1. Set EnableShadow="false"
or
2. Make sure that radwindow.isVisible() === true prior to calling .hide()