How to save treeview xml correctly when I edit it in winfroms?

Question

Let's say. If I have this kind of xml file.

<Main>
  <group name="R1">
    <IsUser>Example</IsUser>
    <IsUser>Example2</IsUser>
  </group>
</Main>

I tried to render into a treeview and removed this part.

<IsUser>Example2</IsUser>

And save it with this code.

        protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
        {
            if (keyData == (Keys.Control | Keys.S))
            {
                ExportToXML(treeView, FileName);
                return true;
            }
            return base.ProcessCmdKey(ref msg, keyData);
        }
        public void ExportToXML(TreeView tree, string filename)
        {
            XmlTextWriter = new XmlTextWriter(filename, System.Text.Encoding.UTF8);
            XmlTextWriter.WriteStartDocument();
            XmlTextWriter.WriteStartElement(treeViewReport.Nodes[0].Text);
            foreach (TreeNode node in tree.Nodes)
            {
                SaveToXML(node.Nodes);
            }
            XmlTextWriter.WriteEndElement();
            XmlTextWriter.Close();
        }

        private void SaveToXML(TreeNodeCollection menu)
        {
            foreach (TreeNode node in menu)
            {
                if (node.Nodes.Count > 0)
                {
                    XmlTextWriter.WriteStartElement(node.Text);
                    SaveToXML(node.Nodes);
                    XmlTextWriter.WriteEndElement();
                }
                else
                {
                    XmlTextWriter.WriteString(node.Text);
                }
            }
        }

but It's saved like this.

<?xml version="1.0" encoding="utf-8"?><Main><group name="R1" ><IsUser >Example</IsUser ></group name="R1" ></Main>

How to save it corretly? Thank you!

-----Edit-----
It should be look like this.

<Main>
  <group name="R1">
    <IsUser>Example</IsUser>
  </group>
</Main>