Ошибка "Object reference not set to an instance of an object" - C#

        XmlDocument doc = new XmlDocument ();
        Audio[] audio = new Audio[98];
        private AudioAdapter GetData()
        {
 
            XmlNodeList title_nodes = doc.GetElementsByTagName ("title");
            XmlNodeList artist_nodes = doc.GetElementsByTagName ("artis");
            XmlNodeList url_nodes = doc.GetElementsByTagName ("url");
            XmlNodeList duration_nodes = doc.GetElementsByTagName ("duration");
 
            for (int i = 0; i < title_nodes.Count; i++) {
                audio [i] = new Audio () {
                    Title = title_nodes [i].InnerText,
                    Artist = artist_nodes [i].InnerText,
                    Url = url_nodes [i].InnerText
                };
            }
 
            AudioAdapter adapter = new AudioAdapter (this, audio);
 
            return adapter;
        }