How to republish odometry in different frame?

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have navigation Odometry, but its some how it is not in the robot body frame. The odometry is being published in world frame, so its not correct. So i need to transform it in the robot body frame as that how should be in the correct way. So I tried to republish the linear velocity in the x axis in the robot body frame, just to check that Im on the correct way, but the code is not working. Here is the ROS node

#include <ros/ros.h>
#include <tf/transform_broadcaster.h>
#include <nav_msgs/Odometry.h>

float linear_x;
ros::Publisher odom_pub;

void poseCallback(const nav_msgs::OdometryConstPtr& msg){

    linear_x = (msg->twist.twist.linear.x );
    nav_msgs::Odometry pose_gt_frame;

        pose_gt_frame.header.frame_id = "world";

    //set the velocity
    pose_gt_frame.child_frame_id = "rexrov2/base_link";
    pose_gt_frame.twist.twist.linear.x = linear_x;

    //publish the message
    odom_pub.publish(pose_gt_frame);

}

int main(int argc, char** argv){
  ros::init(argc, argv, "my_tf_broadcaster");

  ros::NodeHandle node;
  ros::Subscriber sub = node.subscribe("/rexrov2/pose_gt", 10, &poseCallback);
  ros::spin();
  return 0;
};

When run the code I got error

[FATAL] [1635340917.678039503, 15.652000000]: ASSERTION FAILED
    file = /opt/ros/melodic/include/ros/publisher.h
    line = 106
    cond = false
    message = 
[FATAL] [1635340917.680256176, 15.654000000]: Call to publish() on an invalid Publisher
[FATAL] [1635340917.680299807, 15.654000000]:

What can be wrong? Any help? Thanks