Traceback (innermost last):
  Module ZPublisher.Publish, line 119, in publish
  Module ZPublisher.mapply, line 88, in mapply
  Module ZPublisher.Publish, line 42, in call_object
  Module Products.GenericSetup.tool, line 656, in manage_exportSelectedSteps
  Module Products.GenericSetup.tool, line 1117, in _doRunExportSteps
  Module plone.app.contentrules.exportimport.rules, line 433, in exportRules
  Module Products.GenericSetup.utils, line 531, in _exportBody
  Module xml.dom.minidom, line 59, in toprettyxml
  Module Products.GenericSetup.utils, line 441, in writexml
  Module Products.GenericSetup.utils, line 412, in writexml
  Module Products.GenericSetup.utils, line 395, in writexml
AttributeError: 'NoneType' object has no attribute 'encode'