System.ArgumentNullException: Value cannot be null.
Parameter name: String
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt64(String value, NumberStyles options, NumberFormatInfo numfmt)
   at aplicacioneslibre_editoriales_Editorial.Page_Load(Object sender, EventArgs e)