文档空指针异常错误

Question

我遇到了一些空指针异常错误消息的问题。基本上我有一个方法返回Document对象，我的其他方法将使用Document对象作为参数。

以下是我上网的参考资料：

public Document getDocument(LatLng start, LatLng end, String mode) {
    String url = "http://maps.googleapis.com/maps/api/directions/xml?" 
            + "origin=" + start.latitude + "," + start.longitude  
            + "&destination=" + end.latitude + "," + end.longitude 
            + "&sensor=false&units=metric&mode="+mode;

    try {
        HttpClient httpClient = new DefaultHttpClient();
        HttpContext localContext = new BasicHttpContext();
        HttpPost httpPost = new HttpPost(url);
        HttpResponse response = httpClient.execute(httpPost, localContext);
        InputStream in = response.getEntity().getContent();
        DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
        Document doc = builder.parse(in);
        return doc;
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

当我测试URL时，它确实返回了一些东西。但是，当它执行此方法时，它只是继续为Document对象返回null。有人可以向我解释一下代码的Http部分是什么吗？我很困惑。

提前致谢。

已编辑的部分

LatLng fromPosition = new LatLng(1.3564, 103.8311);
            LatLng toPosition = new LatLng(1.3426, 103.9254);

            map = ((SupportMapFragment)getSupportFragmentManager()
                    .findFragmentById(R.id.map)).getMap();
            GMapV2Direction md = new GMapV2Direction();

            Document doc = md.getDocument(fromPosition, toPosition, GMapV2Direction.MODE_DRIVING);
            System.out.println(doc);
            map.addMarker(new MarkerOptions().position(fromPosition).title("Start"));
            map.addMarker(new MarkerOptions().position(toPosition).title("End"));
            ArrayList<LatLng> directionPoint = md.getDirection(doc);
            PolylineOptions rectLine = new PolylineOptions().width(3).color(Color.RED);

            for(int i = 0 ; i < directionPoint.size() ; i++) {          
            rectLine.add(directionPoint.get(i));
            }

            map.addPolyline(rectLine);