AttributeError: 'NoneType' object has no attribute 'items'

[Diogo Matsubara]

OOPS-5c0065e42ff908151f31dcc8d538bb9c raised an AttributeError: 'NoneType' object has no attribute 'items' requesting a preview-diff using the API

   Traceback (most recent call last):
  Module zope.publisher.publish, line 134, in publish
    result = publication.callObject(request, obj)
  Module lazr.restful.publisher, line 211, in callObject
    WebServicePublicationMixin, self).callObject(request, object)
  Module lp.services.webapp.publication, line 499, in callObject
    return mapply(ob, request.getPositionalArguments(), request)
  Module zope.publisher.publish, line 109, in mapply
    return debug_call(obj, args)
   - __traceback_info__: <security proxied lazr.restful._resource.EntryResource instance at 0xda50d50>
  Module zope.publisher.publish, line 115, in debug_call
    return obj(*args)
  Module lazr.restful._resource, line 914, in __call__
    result = self.do_GET()
  Module lazr.restful._resource, line 1547, in do_GET
    media_type = self.handleConditionalGET()
  Module lazr.restful._resource, line 329, in handleConditionalGET
    existing_etag = self.getETag(media_type)
  Module lazr.restful._resource, line 431, in getETag
    etag_cores = self._getETagCores(cache)
  Module lazr.restful._resource, line 1421, in _getETagCores
    name, self._unmarshallField(name, field)[1])
  Module lazr.restful._resource, line 812, in _unmarshallField
    repr_value = marshaller.unmarshall(self.entry, value)
  Module lazr.restful.marshallers, line 542, in unmarshall
    for key, val in value.items())
AttributeError: 'NoneType' object has no attribute 'items'