OpenCV 2.4.8 Ubuntu 14.04 imshow QMetaMethod::invoke Freeze

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I've installed OpenCV 2.4.8 (libopencv-dev) by using apt-get and receive the following message.

init done opengl support available QMetaMethod::invoke: Unable to invoke methods with return values in queued connections.

I ran the program in gdb and it actually seems to stop and freeze on calling cv::waitKey(30); however, the problem only occurs when I call cv::imshow and it seems to be only when imshow is called in a callback function (in a single threaded program that had imshow in a loop did not exhibit the same problems).

Here is the backtrace:

#0  0x00007ffff27f8d7d in nanosleep () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007ffff376d3c8 in ros::ros_wallsleep(unsigned int, unsigned int) ()
   from /opt/ros/indigo/lib/librostime.so
#2  0x00007ffff4890877 in ros::waitForShutdown() () from /opt/ros/indigo/lib/libroscpp.so
#3  0x00007ffff48ac2f8 in ros::MultiThreadedSpinner::spin(ros::CallbackQueue*) ()
   from /opt/ros/indigo/lib/libroscpp.so
#4  0x0000000000412267 in CM::Robot::Robot (this=0x7fffffffd210, handle=..., mod=...)
    at /home/charlie/turtlebot/packages/mapping/src/robot.cc:88
#5  0x000000000041f128 in main (argc=1, argv=0x7fffffffd468)
    at /home/charlie/turtlebot/packages/mapping/src/mapping.cc:32

Here is the output. I added a few cout statements to help decide exactly where the program was freezing.

Before
init done 
opengl support available 
Between
After
Before
QMetaMethod::invoke: Unable to invoke methods with return values in queued connections
Between
Killed

Here is a simplified version of the code that produces the problem: robot.h

# necessary include statements...
(opencv specific includes)
#include <opencv2/core/core.hpp>
#include <opencv2/nonfree/nonfree.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>
#include <highgui.h>

Robot {
 public:
  Robot(ros::NodeHandle& handle, const std::string& mod): node(handle), mode(mod){
   ros::MultiThreadedSpinner threads(3);
   ros::Subscriber odom  = node.subscribe("/odom", 1, &Robot::get_pose, this);
   ros::Subscriber cloud = node.subscribe("/camera/depth/points", 1, &Robot::add_cloud, this);
   threads.spin();
  }
  void get_pose(nav_msgs::Odometry data){
   (...)
  }
  void add_cloud(sensor_msgs::PointCloud2 cloud){
   PointCloud::Ptr cl(new(PointCloud));
   pcl::fromROSMsg(cloud, *cl);
   cl->width = 640;
   cl->height = 480;
   extract_surf_features(cl);
   (...)
  }
  void extract_surf_features(pcl::PointCloud<pcl::PointXYZRGB>::Ptr points){
   cv::Mat image(cur->height, cur->width, CV_8UC3);
   (... get 2d rgb image from point cloud ...)
   cv::SURF surf_extractor(500.0); // Hession threshold = 500.0
   std::vector<cv::KeyPoint> keypoints;
   // actually compute the surf keypoints
   surf_extractor(image, cv::Mat(), keypoints);
   cv::drawKeypoints(image, keypoints, image, cv::Scalar::all(-1), cv::DrawMatchesFlags::DRAW_RICH_KEYPOINTS);
   cout << "Before\n";
   cv::imshow("Image", image);
   cout << "Between\n";
   cv::waitKey(30);
   cout << "After\n";
   (...)
  }
 private:
  ros::NodeHandle node;
  std::string mode;
}

main.cc:

#include <ros/ros.h>
#include <iostream>
#include "robot.h"

using namespace std;

int main(int argc, char** argv){
 ros::init(argc, argv, "PCL_Map");
 if (!ros::master::check()){
  cout << "\033[1;31mERROR: ROS Master not found. Please Start ROS.\n\033[0m";
  return -1;
 }
 ros::NodeHandle node("/map/pcl");

 node.setParam("downsample", 0.04);

 CM::Robot robot(node,"SURF");

 return 0;
}

I have written another very similar program that does not exhibit the freezing behaviors ... (more)