Tree View with Web Part Problem

[Kailash]

I have an User Control and this UC contains a Telerik Tree View control that is nested inside a Panel. Now i put this UC in a webpartzone.Everything work fine but when i close this webpart i got this error at runtime.

VS throws a null exception error:
Microsoft JScript runtime error: 'null' is null or not an object

This is the code where the exception is thrown (line indicated) :

Telerik.Web.UI.RadTreeView._preInitialize=function(_219,_21a){
var _21b=$get(_219);
_21b.scrollTop=_21a; <-- Error
if(Telerik.Web.UI.RadTreeView._isRtl(_21b)){
Telerik.Web.UI.RadTreeView._initializeRtl(_21b);
}
};

Please suggest