TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType' after solar o run-once last

[Jedrzej Nowak]

vagrant@solar-dev:/vagrant$ solar o run-once last

Traceback (most recent call last):
  File "/usr/local/bin/solar", line 10, in <module>
    sys.exit(run())
  File "/vagrant/solar/cli/main.py", line 172, in run
    main()
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/vagrant/solar/cli/orch.py", line 120, in run_once
    wait_report(uid, wait)
  File "/vagrant/solar/cli/orch.py", line 64, in wait_report
    click_report(uid)
  File "/vagrant/solar/cli/orch.py", line 76, in click_report
    report = graph.report_progress(uid)
  File "/vagrant/solar/orchestration/graph.py", line 203, in report_progress
    return report_progress_graph(get_graph(uid))
  File "/vagrant/solar/orchestration/graph.py", line 209, in report_progress_graph
    'total_time': longest_path_time(dg),
  File "/vagrant/solar/orchestration/graph.py", line 119, in longest_path_time
    return end - start
TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType'