title: django, admin template error Caught an exception while rendering: 'NoneType' object has no attribute 'label'
body: <p>good day guys!
  in project, among others, have models:</p>
  
  <pre><code>class Category(models.Model):
  
      name = models.CharField(max_length = 50, blank = False, null = False)
  
      def __unicode__(self):
          return "Category %s" % self.name
  
      class Meta:
          db_table = "categories"
          managed = False
  
  
  class Site(models.Model):
  
      user = models.ForeignKey(User, blank = False, null = False, db_column = "user_id")
      name = models.URLField(verify_exists = True, blank = False, null = False)
      categories = models.ManyToManyField(Category, blank = True, through = "CategorySites", verbose_name = "Category")
  
      def __unicode__(self):
          return self.name
  
      class Meta:
          db_table = "sites"
          managed = False
  
  
  
  class CategorySites(models.Model):
  
      site = models.ForeignKey(Site, blank = False, null = False, db_column = "site_id")
      category = models.ForeignKey(Category, blank = False, null = False, db_column = "category_id")
  
      def __unicode__(self):
          return "Relation between site %s and category %s" % (self.site.name, self.category.name)
  
      class Meta:
          db_table = "categories_sites"
          managed = False
  </code></pre>
  
  <p>so, as you see there is many-to-many relation. generally it work well - i can add and manage models through manage.py shell, or server-side functions.
  I want to enable editing this type of relation on admin site, so, i've added admin model for Sites:</p>
  
  <pre><code> class SiteAdmin(admin.ModelAdmin):
  
      list_display = ('id', 'name')
      list_filter = ('name', 'categories')
  
      fieldsets = (
          (None, {"fields": ("categories",)}),
      )
  
      def queryset(self, request):
          qs = super(SiteAdmin, self).queryset(request)
          if request.user.is_superuser:
              return qs
          else:
              return qs.filter(user = request.user)
  
      def has_change_permission(self, request, obj=None):
          if not obj:
              return True # So they can see the change list page
          if request.user.is_superuser or obj.user == request.user:
              return True
          else:
              return False
  
      has_delete_permission = has_change_permission
  </code></pre>
  
  <p>but when i'm going in admin->sites->Add site (or edit) django throws Caught an exception while rendering: 'NoneType' object has no attribute 'label' .
  how can if fix it?</p>