WrongType b'127.0.0.1' when saving ThemingControlpanel on Py3

[MrTango]

In Python 3 when saving the theming control panel, i get this error:

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module plone.app.theming.browser.controlpanel, line 56, in __call__
  Module plone.app.theming.browser.controlpanel, line 200, in update
  Module plone.registry.recordsproxy, line 60, in __setattr__
  Module plone.registry.registry, line 51, in __setitem__
  Module plone.registry.record, line 82, in _set_value
  Module zope.schema._bootstrapfields, line 291, in validate
  Module zope.schema._field, line 814, in _validate
zope.schema._bootstrapinterfaces.WrongContainedType: ([WrongType(b'127.0.0.1', <class 'str'>, '')], 'value')

[mauritsvanrees]

Duplicate of #179.