Trying to send a JSON with VOLLEY

Question

I have the next method to try to send a JSON to a server.

public void send_json(String access_token, int event_id, ReplyQuiz reply) throws JSONException {

        JSONObject data = jsonobject_from_reply(reply);

        JsonObjectRequest sended_req = new JsonObjectRequest(Request.Method.POST, pathto.send_quiz(access_token, event_id), data,
                new Response.Listener<JSONObject>() {
                    @Override
                    public void onResponse(JSONObject jsonObject) {
                        Log.i("SEND_JSON", "SENDED");
                    }
                }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError volleyError) {
                Log.i("SEND_JSON", "" + volleyError.networkResponse.statusCode);
            }
        });

        volley_queue.add(sended_req);
    }

I always have an error in onErrorResponse.

java.lang.NullPointerException
                at com.nikolay.mm.app.Eventos.EncuestaCuestionariosActivity$8.onErrorResponse(EncuestaCuestionariosActivity.java:394)
                at com.android.volley.Request.deliverError(Request.java:580)
                at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:101)
                at android.os.Handler.handleCallback(Handler.java:730)
                at android.os.Handler.dispatchMessage(Handler.java:92)
                at android.os.Looper.loop(Looper.java:176)
                at android.app.ActivityThread.main(ActivityThread.java:5419)
                at java.lang.reflect.Method.invokeNative(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:525)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
                at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
                at dalvik.system.NativeStart.main(Native Method)

pathto.send_quiz(access_token, event_id) returns a string with an URL.

data returns a JSONObject with the next method.

public JSONObject jsonobject_from_reply(ReplyQuiz reply) throws JSONException {
            JSONObject data = new JSONObject();
              JSONArray reply_arr = new JSONArray();
            JSONObject reply_arr_obj = new JSONObject();
            reply_arr_obj.put("nombre", reply.getNombre());
            reply_arr_obj.put("unidad_negocio", reply.getUnidad_negocio());
            reply_arr_obj.put("area_negocio", reply.getArea_negocio());
            reply_arr_obj.put("area_soporte", reply.getArea_soporte());

            reply_arr.put(reply_arr_obj);

            JSONArray answer_arr = new JSONArray();
            for(int i = 0; i < reply.getReplys().size(); i++){
                JSONObject answer_arr_obj = new JSONObject();
                answer_arr_obj.put("answer_id", reply.getReplys().get(i).getAnswer_id());
                answer_arr_obj.put("value", reply.getReplys().get(i).getValue());
                answer_arr.put(respuesta_arr_obj);
            }
            data.put("reply", reply_arr);
            data.put("answer", answer_arr);

            return data;
        }

Where I have the problem? Thanks