title: DjangoFilterBackend not working with my models (manytomany)
body: <p>I have a Django REST project in which I used the DjangoFilterBackend from django-rest-framework-filters<a href="https://github.com/philipn/django-rest-framework-filters" rel="nofollow">github link</a>.</p>
  
  <p>This backend works well for some of my models which has ForeignKey &amp; ManyToMany relations, however, it just doesn't work for some other models and always gives me an <code>AttributeError: 'NoneType' object has no attribute 'verbose_name'</code>.
  my model:</p>
  
  <pre><code>from django.db import models
  from authuser.models import AbstractUser, BaseUserManager
  from corporation.models import Corporation
  
  
  class Organization(AbstractUser):
      company = models.ForeignKey(Corporation, verbose_name='company',
                              related_name='organizations',
                              null=True, on_delete=models.SET_NULL)
      contact_name = models.CharField(max_length=50, verbose_name='contact_name', default='')
      contact_phone = models.CharField(max_length=50, null=True, verbose_name='contact_phone')
      contact_email = models.EmailField(max_length=50,     verbose_name='contact_email', default='')
      about_us = models.TextField(null=True, verbose_name='about_us')
      info_completed = models.BooleanField(default=False,     verbose_name='info_completed')
  
      objects = BaseUserManager()
  
      class Meta:
          ordering = ('date_joined',)
  </code></pre>
  
  <p>my views:</p>
  
  <pre><code>class OrganizationDetailView(generics.RetrieveUpdateDestroyAPIView):
      serializer_class = OrganizationSerializer
      queryset = Organization.objects.all()
  
  
  
  class OrganizationListCreateView(generics.ListCreateAPIView):
      queryset = Organization.objects.all()
      permission_classes = ()
  
      def get_serializer_class(self):
          if self.request.method == "POST":
              return CreateOrganizationSerializer
          return OrganizationSerializer
  
      filter_backends = (
          rest_backends.DjangoFilterBackend,
          filters.SearchFilter,
          filters.OrderingFilter,
      )
      filter_fields = (
          'contact_name',
      )
      filter_class = OrganizationFilter
      search_fields = (
          # 'company',
                       'contact_name',
                       'contact_email', 'contact_phone',
                       'description',
      )
      ordering = ('date_joined')
  </code></pre>
  
  <p>my filterset:</p>
  
  <pre><code>class OrganizationFilter(filters.FilterSet):
      company = filters.RelatedFilter(CorporationFilter, name='company')
      description = filters.AllLookupsFilter(name='description')
      contact_name = filters.AllLookupsFilter(name='contact_name')
      contact_phone = filters.AllLookupsFilter(name='contact_phone')
      contact_email = filters.AllLookupsFilter(name='contact_email')
  
      class Meta:
          model = Organization
          fields = (
              'company',
              'description', 'contact_name', 'contact_phone',
              'contact_email',
          )
  </code></pre>
  
  <p>my serializer:</p>
  
  <pre><code>class OrganizationSerializer(serializers.ModelSerializer):
      company = serializers.PrimaryKeyRelatedField(queryset=Corporation.objects.all())
  
      class Meta:
          model = Organization
          fields = ('id', 'company',
                    'contact_name', 'contact_email',
                    'contact_phone', 'info_completed')
  </code></pre>
  
  <p>The CRUD operations are fine. It just gives me the attribute error when listing organizations:</p>
  
  <pre><code>Creating test database for alias 'default'...
  E.........
  ======================================================================
  ERROR: test_list_organizations (organization.tests.SimpleTest)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "F:\landi-django\organization\tests.py", line 73, in test_list_organizati
  ons
      response = client.get('/organizations?contact_name=Tingtao')
    File "F:\landi-django\env\lib\site-packages\rest_framework\test.py", line 160,
   in get
      response = super(APIClient, self).get(path, data=data, **extra)
    File "F:\landi-django\env\lib\site-packages\rest_framework\test.py", line 86,
  in get
      return self.generic('GET', path, **r)
    File "F:\landi-django\env\lib\site-packages\rest_framework\compat.py", line 18
  9, in generic
      return self.request(**r)
    File "F:\landi-django\env\lib\site-packages\rest_framework\test.py", line 157,
   in request
      return super(APIClient, self).request(**kwargs)
    File "F:\landi-django\env\lib\site-packages\rest_framework\test.py", line 109,
   in request
      request = super(APIRequestFactory, self).request(**kwargs)
    File "F:\landi-django\env\lib\site-packages\django\test\client.py", line 440,
  in request
      six.reraise(*exc_info)
    File "F:\landi-django\env\lib\site-packages\django\core\handlers\base.py", lin
  e 111, in get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "F:\landi-django\env\lib\site-packages\django\views\decorators\csrf.py",
  line 57, in wrapped_view
      return view_func(*args, **kwargs)
    File "F:\landi-django\env\lib\site-packages\django\views\generic\base.py", lin
  e 69, in view
      return self.dispatch(request, *args, **kwargs)
    File "F:\landi-django\env\lib\site-packages\rest_framework\views.py", line 407
  , in dispatch
      response = self.handle_exception(exc)
    File "F:\landi-django\env\lib\site-packages\rest_framework\views.py", line 404
  , in dispatch
      response = handler(request, *args, **kwargs)
    File "F:\landi-django\env\lib\site-packages\rest_framework\generics.py", line
  311, in get
      return self.list(request, *args, **kwargs)
    File "F:\landi-django\env\lib\site-packages\rest_framework\mixins.py", line 40
  , in list
      instance = self.filter_queryset(self.get_queryset())
    File "F:\landi-django\env\lib\site-packages\rest_framework\generics.py", line
  144, in filter_queryset
      queryset = backend().filter_queryset(self.request, queryset, self)
    File "F:\landi-django\env\lib\site-packages\rest_framework_filters\backends.py
  ", line 45, in filter_queryset
      _filter = filter_class(request.QUERY_PARAMS, queryset=queryset)
    File "F:\landi-django\env\lib\site-packages\rest_framework_filters\filterset.p
  y", line 61, in __init__
      f = self.filter_for_field(field, filter_.name)
    File "F:\landi-django\env\lib\site-packages\django_filters\filterset.py", line
   400, in filter_for_field
      'label': capfirst(f.verbose_name),
  AttributeError: 'NoneType' object has no attribute 'verbose_name'
  
  ----------------------------------------------------------------------
  </code></pre>
  
  <p>I also tried commenting line 400 in django_filters\filterset.py, but it gives me more errors. 
  Many thanks for your help!</p>