Unable to access publisher in nodelet

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I am writing a ROS nodelet wrapper class for my existing ROS node. My nodelet is processing image data and publishing an output image with bounding box. I am getting the following error which I assume means that I am publishing before initializing the publisher?!? I have initialized the image publisher in header file. How else I am supposed to initialize it? Can't my nodelet access the publisher? Error -

[FATAL] [1679000699.594996126]: ASSERTION FAILED
    file = /opt/ros/noetic/include/ros/publisher.h
    line = 71
    cond = false
    message = 
[FATAL] [1679000699.595018289]: Call to publish() on an invalid Publisher
[FATAL] [1679000699.595023792]: 
[FATAL] [1679000700.700172311]: Failed to load nodelet '/TestNodelet` of type `test_detect/TestNodelet` to manager `cam_nodelet_manager'

Following is my header file test.h:

// << All includes >>

namespace test {
class TestNode {
 public:
  TestNode(const std::string& cam);
  static TestNode& GetInstance(const std::string& cam = "");
  inline const ros::Publisher& GetImagePub(void) const {return imagePub;}
  bool init();
  const cv::Mat& Filter(const cv::Mat& image);
  const cv::Mat& DT(const cv::Mat& image);    
 private:
  ros::NodeHandle nh;
  ros::Subscriber imageSub;
  ros::Publisher imagePub;
  void InputImageCb(const sensor_msgs::Image::ConstPtr& msg);
  const std::string cam;
};
}

My ROS node looks like the following test_node.cpp:

#include <test/test_node.h>

namespace test {
TestNode::TestNode(const std::string& cam)
    : cam(cam), nh(){};
bool TestNode::init() {
  imageSub = nh.subscribe("/camera/image_raw", 10, &TestNode::InputImageCb, this);
  imagePub = nh.advertise<sensor_msgs::Image>("/camera/final_image", 100);
  return true;
}

void TestNode::InputImageCb(const sensor_msgs::Image::ConstPtr& msg) {
  cv_bridge::CvImageConstPtr cvPtr = cv_bridge::toCvShare(msg, sensor_msgs::image_encodings::BGR8);
  TestNode::GetInstance().SetCurrentImage(cvPtr->image);
  }

const cv::Mat& TestNode::Filter(const cv::Mat& image) { 
  // code
  return mask;
}

const cv::Mat& TestNode::DT(const cv::Mat& image) {
  // code
  return image;
}

}

Now I am trying to wrap the test class in a nodelet class in the following way (test_nodelet.cpp) -

#include "test/test_node.h"
#include "nodelet/nodelet.h"
#include "pluginlib/class_list_macros.h"

namespace test {
class TestNodelet : public nodelet::Nodelet {
 public:
  TestNodelet(){};
  ~TestNodelet() {}

 private:
  boost::shared_ptr<TestNode> node_;

  virtual void onInit() {
    ros::NodeHandle& pnh = this->getMTPrivateNodeHandle();   
    std::string cam;
    pnh.getParam("camera_name", cam);
    node_ = boost::make_shared<TestNode>(cam);
    if (node_->init()) {
      NODELET_INFO_STREAM("Initialized");
    }
    Loop(cam);
  }

  static void Loop(const std::string& cam) {
    TestNode& node_ = node_.GetInstance(cam);

    while (ros::ok()) {
      ros::spinOnce();
        const cv::Mat& mask = node_.Filter(currentImage); // function in node class to filter
        const cv::Mat& bbox = node_.DT(mask); // function to publish bounding box
        sensor_msgs::ImagePtr msg = cv::bridge::CvImage(std_msgs::Header(), "mono8", bbox).toImageMsg();
        const ros::Publisher& imagePub = node_.GetImagePub();
        imagePub.publish(msg);
    }
  }
};
}

PLUGINLIB_EXPORT_CLASS(test::TestNodelet, nodelet::Nodelet);