I haven’t been able to reproduce this on my site(s), but I have looked at a number of kiwitrees hosting client sites and some do display the same issue.
Looking at the code referred to I’m fairly certain I can see what combination of settings might cause it, so hopefully the attached replacement file will fix it. Please let me know after you’ve used it for a couple of days at least. It may take that long as my suspicion is the trigger (not the cause) is a visit from a search engine)
Replace the file kiwitrees/library/WT/MenuBar.php with the one in the attached zip folder.
Delete all existing errors (or at least the ones related to this error).
Check the logs over a few days to see of it recurs.