[Buildbot] #2951: AttributeError: 'NoneType' object has no attribute 'append'

[Buildbot trac]

#2951: AttributeError: 'NoneType' object has no attribute 'append'
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Reporter:  Ben       |       Owner:  tardyp
    Type:  defect    |      Status:  assigned
Priority:  critical  |   Milestone:  0.9.0
 Version:  master    |  Resolution:
Keywords:            |
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Comment (by tardyp):

 I wasn't able to reproduce, but given the tracebacks, I think this PR
 should be able to circumvent the problem.

 https://github.com/buildbot/buildbot/pull/1296

 The idea of dustin is probably also working, but I think it is a little
 bit more complicated

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Ticket URL: <http://trac.buildbot.net/ticket/2951#comment:7>
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