Get model state in gazebo does not exist

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I want to read the position data from the real robot and reflect it in gazebo and read the sensor data from gazebo and do some calculations with these data. I have the following code:

in the constructor

    n_priv_.param<std::string>("model_name", model_name , "quadrotor");
    nav_msgs_sub = n_.subscribe("/pose",1, &ModelState::positionCallback, this);
    model_state_pub = n_.advertise<gazebo_msgs::ModelState>("/set_model_state", 1);

and in the position callback funtion I fill the msg by

void positionCallback (const nav_msgs::Odometry::ConstPtr& msg )
 {

geometry_msgs::Pose start_pose;
    start_pose.position.x = msg->pose.pose.position.x;
    start_pose.position.y = msg->pose.pose.position.y;
    start_pose.position.z = msg->pose.pose.position.z;
    start_pose.orientation.x = msg->pose.pose.orientation.x;
    start_pose.orientation.y = msg->pose.pose.orientation.y;
    start_pose.orientation.z = msg->pose.pose.orientation.z;
    start_pose.orientation.w = msg->pose.pose.orientation.w;

    geometry_msgs::Twist start_twist;
    start_twist.linear.x =msg->twist.twist.linear.x;
    start_twist.linear.y = msg->twist.twist.linear.y;
    start_twist.linear.z = msg->twist.twist.linear.z;
    start_twist.angular.x = msg->twist.twist.angular.x;
    start_twist.angular.y =msg->twist.twist.angular.y;
    start_twist.angular.z = msg->twist.twist.angular.z;

    gazebo_msgs::ModelState modelstate;
    modelstate.model_name = model_name;
    modelstate.reference_frame = (std::string) "world";
    modelstate.pose = start_pose;
    modelstate.twist = start_twist;

model_state_pub.publish(modelstate);

  }

In the main I have the following

  ros::init(argc, argv, "ModelState");
  ros::NodeHandle n;
 ModelState lstopc(n);
 while(ros::ok())
 {
 ros::spinOnce();
  }

The problem is that when I run gazebo ( the command line or the shell ) It shows me an error that says Updating model state : model [my_model_name] does not exist ?