Create xml node with innertext from textbox using data binding

I want to add another list node in product node.

Product.xml file have:

<products>
  <product>
    <list>first</list>
    <list>second</list>
  </product>  
</products>

Code for this is:

<Grid.DataContext>
    <XmlDataProvider x:Name="xmlData" Source="Product.xml" XPath="products/product"/>
</Grid.DataContet>

And in addItem_Click_1:

private void addItem_Click_1(object sender, RoutedEventArgs e)
    {
        try
        {
            string source = xmlData.Source.AbsoluteUri;

            XmlDocument doc = xmlData.Document;

            // Get a handle on the root node.
            XmlNode root = doc.SelectSingleNode("//product");
            XmlNode newitem = doc.CreateElement("item");
            newitem.InnerText = itemTextBox.Text;
            root.AppendChild(newitem);

            // Save the changes.
            xmlData.Document.Save(source);
            MessageBox.Show("Successful");
        }
        catch (Exception d)
        {
            MessageBox.Show(d.Message);
        }
    }

This shows output as:

The operation is not supported for a relative URI.

Product.xml file is:

How can i solve this?Please anyone help