I tried making a complete example which still gives me the 'null' is null or not an object-error:
<%@ Page Language="C#" %>
<%@ Register TagPrefix="telerik" Namespace="Telerik.Web.UI" Assembly="Telerik.Web.UI" %>
<html xmlns="
http://www.w3.org/1999/xhtml">
<head>
<title>Untitled Page</title>
</head>
<body onload="enableTabStrip();">
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<script type="text/javascript" language="javascript">
function enableTabStrip()
{
var myTabStrip = $find("<%= RadTabStrip1.ClientID %>");
myTabStrip.enable();
}
</script>
<telerik:RadTabStrip ID="RadTabStrip1" runat="server">
<Tabs>
<telerik:RadTab runat="server" Text="Root RadTab1" />
<telerik:RadTab runat="server" Text="Root RadTab2" />
<telerik:RadTab runat="server" Text="Root RadTab3" />
</Tabs>
</telerik:RadTabStrip>
</form>
</body>
</html>