Iteadlib Nextion Slider giving error, how can solve this?

I'm using Itead Nextion with Arduino. I added a slider in Nextion display. But I can't get value from the slider and I am getting "recvRetNumber err" this error. How can I solve this? Can you help me? Also I'm using Nextion Stable version (0.7.0).

My codes as follows:

#include <ArduinoJson.h>
#include "Nextion.h"
#include "NexButton.h"
#include "NexText.h"
#include "NexSlider.h"
#include <SoftwareSerial.h>
SoftwareSerial serial(6, 7);

NexButton BtnChangeRotation = NexButton(0, 2, "b0");
NexSlider SldPower = NexSlider(0, 1, "h0");

int drection = 0;
int pwr = 0;

char buffer[200];
DynamicJsonBuffer jsonBuffer(512);
JsonObject& root = jsonBuffer.createObject();
char jsonChar[100];

NexTouch *nex_listen_list[] =
{
  &BtnChangeRotation,
  &SldPower,
  NULL
};

void SldPower_Callback(void *ptr)
{
  SldPower.getValue(pwr);

  root["direction"] = drection;
  root["power"] = pwr;
  root.printTo((char*)jsonChar, root.measureLength() + 1);

  Serial.println(jsonChar);
  serial.write(jsonChar);
}

void setup(void)
{
  Serial.begin(9600);
  serial.begin(9600);

  root["direction"] = 0;
  root["power"] = 0;


  nexInit();

  BtnChangeRotation.attachPop(BtnChangeRotation_Callback, &BtnChangeRotation);
  SldPower.attachPop(SldPower_Callback);
}

void loop(void)
{
  nexLoop(nex_listen_list);
}