[Spambayes] pop3proxy: find message by id, show clues, error

[Harri Pesonen]

      500 Server error

Traceback (most recent call last):

  File "D:\SRC\spambayes-1.0a4\spambayes\Dibbler.py", line 398, in found_terminator
    getattr(plugin, name)(**params)

  File "D:\SRC\spambayes-1.0a4\spambayes\ProxyUI.py", line 425, in onShowclues
    message = state.unknownCorpus.get(key).getSubstance()

AttributeError: 'NoneType' object has no attribute 'getSubstance'