What version of toga are you running? I'm on Ubuntu with Python 2.7.6, toga v 0.1.0 and cassowary 0.5 and your oneliner works fine.

Edit: In fact, OptionContainer is just 15 days old, so I think you've got an old version: https://github.com/pybee/toga/commit/262efbfc00fae0a96e09666...