Cách sửa lỗi Object reference not set to an instance of an object trong Xml

[thangemhamhochoi]

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private void btnXml_Click(object sender, EventArgs e)
        {
            try
            {
                XmlDocument Source = new XmlDocument();
                String filename = "Data.xml";
                //Source.Load(filename);
                FileStream fs = new FileStream(filename, FileMode.Open);
                XmlTextReader xtr = new XmlTextReader(fs);
                string languagename = "";
                string groupname = "";
                clWord word = new clWord();
                DataTable xml = word.Xml_List();
                if (xml.Rows.Count > 0)
                {
                    for (int i = 1; i <= xml.Rows.Count; i++)
                    {
                        languagename = xml.Rows[i]["languageName"].ToString();
                        groupname = xml.Rows[i]["groupName"].ToString();
                        if (xtr.MoveToContent() == XmlNodeType.Element && xtr.Name == languagename)
                        {
                            if (xtr.MoveToContent() == XmlNodeType.Element && xtr.Name == groupname)
                            {
                                XmlNode group = Source.SelectSingleNode("/" + languagename + "/" + groupname);
                                XmlElement Element;
                                Element = Source.CreateElement("WordID");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["wordID"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Kanji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["kanji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Hiragana");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["hiragana"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Romaji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["romaji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Vietnamese");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["imi"]);
                                group.AppendChild(Element);
                                Source.Save(filename);
                            }
                            else
                            {
                                XmlNode root = Source.SelectSingleNode("/Dictionary/" + languagename);
                                XmlElement group = Source.CreateElement("" + groupname);
                                root.AppendChild(group);
                                XmlElement Element;
                                Element = Source.CreateElement("WordID");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["wordID"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Kanji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["kanji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Hiragana");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["hiragana"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Romaji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["romaji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Vietnamese");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["imi"]);
                                group.AppendChild(Element);
                                Source.Save(filename);
                            }
                         }
                        else
                        {
                            XmlNode root = Source.SelectSingleNode("Dictionary");
                            XmlElement lg = Source.CreateElement("" + languagename);
                            root.AppendChild(lg);  //Báo lỗi ở đây
                            Source.Save(filename);
                            if (xtr.MoveToContent() == XmlNodeType.Element && xtr.Name == groupname)
                            {
                                XmlNode group = Source.SelectSingleNode("/" + languagename + "/" + groupname);
                                XmlElement Element;
                                Element = Source.CreateElement("WordID");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["wordID"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Kanji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["kanji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Hiragana");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["hiragana"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Romaji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["romaji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Vietnamese");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["imi"]);
                                group.AppendChild(Element);
                                Source.Save(filename);
                            }
                            else
                            {
                                XmlNodeList root2 = Source.GetElementsByTagName(languagename);
                                XmlElement group = Source.CreateElement("" + groupname);
                                root2[0].InnerText = groupname;
                                XmlElement Element;
                                Element = Source.CreateElement("WordID");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["wordID"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Kanji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["kanji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Hiragana");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["hiragana"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Romaji");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["romaji"]);
                                group.AppendChild(Element);
                                Element = Source.CreateElement("Vietnamese");
                                Element.InnerText = Convert.ToString(xml.Rows[i]["imi"]);
                                group.AppendChild(Element);
                                Source.Save(filename);
                            }
                        }
                    }
                }
                MessageBox.Show("Import thành công");
            }
            catch (Exception)
            {
                MessageBox.Show("Lỗi không thể Import dữ liệu được");
            }
        }

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