AsyncFileUpload inside a TabContainer produces a very strange error ?

I have really stucked at a big problem using the AsyncFileUpload inside a TabContainer. It seems that when I have put the AsyncFileUploadControl inside a TabPanel in the TabContainer and now switch tab manually by clicking I receive an error that is another codefile than the .cs file. I cant understand what is causing this error ?
The Error that is highlighted in the code will be this: 
a._innerTB.style.width=a._inputFile.offsetWidth-107+"px";
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="page2.aspx.cs" Inherits="_Default" %> <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> &l ...