Java将String转换为XML文档Null指针StringReader中的异常

Question

从http url获取xml文档，并且打印文档完美无缺（System.out.println（xmlResult）），但是当我将字符串转换为xml文档时，它会产生空指针异常

Java代码

public static void main(String[] args) throws MalformedURLException, IOException, ParserConfigurationException, SAXException {
        DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder documentBuilder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
        URL url = new URL("http://ops.epo.org/3.1/rest-services/published-data/publication/epodoc/US7039403/biblio");

        HttpURLConnection epo = (HttpURLConnection) url.openConnection();
        epo.setRequestMethod("GET");
        epo.setRequestProperty("ContentType", "text/xml;charset=\"utf-8\"");
        epo.setRequestProperty("Accept", "text/xml");

        InputStream inputstream = epo.getInputStream();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputstream,"UTF-8"));

            String xmlResult = "";
            while ((xmlResult = bufferedReader.readLine()) != null) {                
            }           
            InputSource inputSource = new InputSource();
            inputSource.setCharacterStream(new StringReader(xmlResult));
            Document xmlDocument = documentBuilder.parse(inputSource);

            System.out.println("Root element :" + xmlDocument.getDocumentElement().getNodeName());        
    }

异常详情

Exception in thread "main" java.lang.NullPointerException
    at java.io.StringReader.<init>(StringReader.java:50)
    at ExtractBiblio.Teser.main(Teser.java:62) 
Line:inputSource.setCharacterStream(new StringReader(xmlResult));

帮我解决。