web based FTP client

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C#

I downloaded the source code from :
A web based FTP client[^]

i was able to connect to my ftp server and see the list of the directories but as soon as i select a file and click download i get this error :

NullreferenceException was unhanled by user

the error in this line :

protected void grdDetailView_RowCommand(object sender, GridViewCommandEventArgs e)
    {

        byte[] _downFile = FTP.DownloadFileFromFtp(e.CommandArgument.ToString());
        string _fname = e.CommandArgument.ToString();
        Response.ContentType = "application/" + Server.MapPath("filename.ext");

        Response.ContentType = "application/" + _fname.Split('.')[1];
        Response.AddHeader("Content-disposition", "attachment; filename=" + _fname);
        Response.OutputStream.Write(_downFile, 0, _downFile.Length);
        Response.End();


    }

Please i will appreciate any help I'm a student and i have a project to do and i want to integrate this ftp to my project website asp.net.

Thanks in advance.