Should be a fix for https://en.wikipedia.org/wiki/Wikipedia_talk:AutoWikiBrowser/Bugs#NullReferenceException_in_TraceManager.RemoveListener

git-svn-id: svn://svn.code.sf.net/p/autowikibrowser/code@9368 5227b50d-9349-4574-b055-4b0a7525e11c