viewer.spinOnce () failed normal_estimation_using_integral_images.cpp

Testing normal_estimation_using_integral_images.cpp, result showed for a sec, but failed right away at line viewer.spinOnce ();:

    while (!viewer.wasStopped ())
    {
      viewer.spinOnce ();
    }

with the following ERROR message:

X connection to p% broken (explicit kill or server shutdown).
[1] + Done                       "/usr/bin/gdb" --interpreter=mi --tty=${DbgTerm} 0<"/tmp/Microsoft-MIEngine-In-ceuxubyq.vps" 1>"/tmp/Microsoft-MIEngine-Out-iriujkvs.ttm"

Can anybody please help?

Cheers