title: Django: object has no attribute 'object'
body: <p>I have to implement several forms, therefore I need the combination of <code>SingleObjectMixin</code>, <code>TemplateView</code>. I always receive <code>'AssignAttendee' object has no attribute 'object'</code>. Do you see why I get that error?</p>
  
  <pre><code>class AssignAttendee(SuccessMessageMixin, SingleObjectMixin, TemplateView):
      template_name = 'attendees/front/assign_attendee.html'
      success_message = _("Attendee has been successfully updated.")
  
      def get_object(self):
          return get_object_or_404(
              Attendee,
              ticket_reference=self.kwargs['ticket_reference'],
              ticket_code=self.kwargs['ticket_code'],
          )
  
      @cached_property
      def attendee_form(self):
          return AssignAttendeeForm(
              prefix='attendee',
              data=self.request.POST or None,
              # instance=self.attendee_contact,
          )
  
      def get_context_data(self, **kwargs):
          context = super().get_context_data(**kwargs)
          context = {
              'attendee_form': self.attendee_form,
          }
  </code></pre>