HI mjcairns,
I get the same error message, in 2.80, but it’s still working.
In 2.79 I don’t get any error messages, but for the rest the behaviour appears to be the same.
It looks like something’s wrong in the code of the 3D Print Toolbox (due to the changed Python API, probably…).
But as long as it works, who cares? If it doesn’t work for you, maybe contact Campbell Barton (there is a ‘Report a Bug’ button in the Addons section of the Preferences).
