Unable to get value of the property 'toLowerCase': object is null or undefined

HOW-TO:
Open a modal RadWindow inside another RadWindow (or iframe) in IE9 and IE10 and avoid the JavaScript error this can cause because of a browser bug.

Note: This also applies to the server RadAlert(), RadConfirm() and RadPrompt() methods and the radopen(), radalert(), radconfirm() and radprompt() client-side methods when called from the server in the Sys.Application.Load event.

This can be solved by adding  a new method OnClientBeforeShow="fixIE" of
RadWindowManager


  <telerik:RadWindowManager ID="radWinManager"
            runat="server" DestroyOnClose="True" OnClientBeforeShow="fixIE"
            KeepInScreenBounds="true">
  </telerik:RadWindowManager>

Function

  function fixIE() {
       document.documentElement.focus();
    }