btn disable but not enable

[RakeshKumar]

when i click btn control disable btn using javascript then btn click event call. inside btn click event i write code which gives me report in excel/pdf format for that i use method ExportToHttpResponse(ExportFormatType.Excel, Response, true, "Report Name") which gives me exception so after this method or finally block when i write btn.enable=true; or use javascript then not executed. exception is Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack

i.e. i have to enable btn when report is generated. as this method get executed ExportToHttpResponse() i got report so after this method i have to enable btn control

protected void Page_Load(object sender, EventArgs e)
{
    btnExcel.Attributes.Add("onclick", " this.disabled = true; " + ClientScript.GetPostBackEventReference(btnExcel, null) + ";");
}
 
 
protected void btnExcel_Click(object sender, EventArgs e)
{
   view_report();
    try
    {
       Rpt.ExportToHttpResponse(ExportFormatType.Excel, Response, true, " Reportname");
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
        ex = null;
    }
    finally
    {                  
       btnExcel.Enabled = true;
        ScriptManager.RegisterClientScriptBlock(Page, this.GetType(), "CallJSFunction", "myEnablefunction()", true);
    }
}

js function is

function myEnablefunction()
{ 
var BtnEx=document.getElementById("ctl00$ContentPlaceHolder$btnExcel");
BtnEx.disabled=false;
 
}