java - Add elements to existing XML file using Java

Question

I have an existing blank XML file like this:

<root>
<!--Some Comment goes here>
</root>

Now I want have a GUI which has 1 textfield and 1 textarea which takes some string(title and content) and upon clicking the button 'Save' it adds them to the XML file under the element. I have created root in a separate GUI and saved into 'sample.xml' and upon clicking a button, the content add window comes up, where I open the same XML file and want to add the items into that. But every time I do so, I am getting a 'Null Pointer Exception' error. The following is my code. Please tell me where I am going wrong:

Code in the main GUI:

JButton btnNewButton = new JButton("Add New");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                //File f = new File("sample.xml");
                DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
                DocumentBuilder docBuilder;
                try {
                    Writer output = new BufferedWriter(new FileWriter("sample.xml"));
                    docBuilder = dbfac.newDocumentBuilder();
                    Document doc = docBuilder.newDocument();

                    Element root = doc.createElement("root");
                    doc.appendChild(root);

                    Comment comment = doc.createComment("Just a thought");
                    root.appendChild(comment);

                    TransformerFactory transfac = TransformerFactory.newInstance();
                    Transformer trans = transfac.newTransformer();
                    trans.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
                    trans.setOutputProperty(OutputKeys.INDENT, "yes");

                    StringWriter sw = new StringWriter();
                    StreamResult result = new StreamResult(sw);
                    DOMSource source = new DOMSource(doc);
                    trans.transform(source, result);
                    String xmlString = sw.toString();

                    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("sample.xml", true)));
                    out.println(xmlString);
                    out.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (ParserConfigurationException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (TransformerException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

Code in the secondary GUI to add elements:

JButton btnSave = new JButton("Save");
        btnSave.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

                DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
                dbfac.setIgnoringComments(true);
                DocumentBuilder docBuilder;
                try {
                    docBuilder = dbfac.newDocumentBuilder();
                    Document doc = docBuilder.parse("sample.xml");

                    NodeList rt = doc.getElementsByTagName("root");

                    Element child = doc.createElement("content");
                    child.setAttribute("title", textField.getText().toString());
                    rt.item(0).appendChild(child);

                    Text text = doc.createTextNode(textArea.getText().toString());
                    child.appendChild(text);

                    TransformerFactory transfac = TransformerFactory.newInstance();
                    Transformer trans = transfac.newTransformer();
                    trans.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
                    trans.setOutputProperty(OutputKeys.INDENT, "yes");

                    StringWriter sw = new StringWriter();
                    StreamResult result = new StreamResult(sw);
                    DOMSource source = new DOMSource(doc);
                    trans.transform(source, result);
                    String xmlString = sw.toString();

                    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("sample.xml", true)));
                    out.println(xmlString);
                    out.close();


                    System.out.println(xmlString);
                } catch (ParserConfigurationException e) {
                    e.printStackTrace();
                } catch (TransformerConfigurationException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (TransformerException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (FileNotFoundException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (SAXException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }



            }
        });