[/rviz] wants topic /visualization_markers to have datatype/md5sum

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I am trying to publish and visualize MarkerArray.

I am calling publish_marker_array function from inside my main function -

void publish_marker_array(ros::NodeHandle &nh, vector<pair<size_t, size_t> >&frontiers, costmap_2d::Costmap2D* &global_costmap_) {

    cout << "INSIDE THE PUBLISH_MARKERS FUNCTION! " << endl;

    size_t  sz = frontiers.size();

    visualization_msgs::MarkerArray frontier_marker_array;

    for(int i =0 ;i  < sz ; i++){

        visualization_msgs::Marker marker;

        marker.header.frame_id = "map";
        marker.header.stamp = ros::Time();

        cout << "namespace: " << nh.getNamespace() << endl;

        marker.ns = nh.getNamespace();

        marker.id = 1;
        marker.type = visualization_msgs::Marker::SPHERE;
        marker.action = visualization_msgs::Marker::ADD;


        marker.pose.position.x =  2000;
        marker.pose.position.y =   2000;
        marker.pose.position.z = 1; 
        marker.pose.orientation.x = 0.0;
        marker.pose.orientation.y = 0.0;
        marker.pose.orientation.z = 0.0;
        marker.pose.orientation.w = 1.0;

        marker.lifetime = ros::Duration(1.0);

        marker.scale.x = 1;
        marker.scale.y = 1;
        marker.scale.z = 1.0;

        marker.color.a = 1.0; // Don't forget to set the alpha!

        marker.color.r = 1.0;
        marker.color.g = 0.0;
        marker.color.b = 0.0;


        frontier_marker_array.markers.push_back(marker);

    }





    ros::Time start = ros::Time::now(); 
    ros::Duration del(20.0);
    ros::Time end = start + del ;

    cout << "Loop starts at: " << ros::Time::now().toSec() << endl;

    while(ros::Time::now() < start + del) {


        ros::Publisher vis_pub = nh.advertise<visualization_msgs::MarkerArray>( "visualization_markers", 10 );
        //marker.id = ++id;
        vis_pub.publish(frontier_marker_array);
        //ros::Duration(0.5).sleep();
        //cout << "Current time: " << ros::Time::now().toSec() << endl;


    }

    cout << "Loop ends at: " << ros::Time::now().toSec() << endl;


}

On running the node, I am getting the following error-

[ERROR] [1621076826.279428345, 756.115000000]: Client [/rviz] wants topic /explore_node/visualization_markers to have datatype/md5sum [visualization_msgs/Marker/4048c9de2a16f4ae8e0538085ebf1b97], but our version has [visualization_msgs/MarkerArray/d155b9ce5188fbaf89745847fd5882d7]. Dropping connection.

What could be the possible reasons for the above mentioned error?