stack trace on exit

[scottdraves]

open a groovy notebook, run %%python then shutdown via control-c:

Shutdown this notebook server (y/[n])? y
[C 23:46:41.139 NotebookApp] Shutdown confirmed
[I 23:46:41.140 NotebookApp] Shutting down 2 kernels
Exception in thread "Thread-1" java.lang.NullPointerException
        at com.twosigma.beakerx.kernel.comm.Comm.close(Comm.java:180)
        at java.util.concurrent.ConcurrentHashMap$ValuesView.forEach(ConcurrentHashMap.java:4707)
        at com.twosigma.beakerx.kernel.Kernel.closeComms(Kernel.java:140)
        at com.twosigma.beakerx.socket.KernelSocketsZMQ.close(KernelSocketsZMQ.java:218)
        at com.twosigma.beakerx.socket.KernelSocketsZMQ.run(KernelSocketsZMQ.java:177)