title: Django SessionWizardView ModelMultipleChoiceField issue
body: <p>I am implementing <strong>formtools.wizard.views</strong> and on first form i have a <strong>ModelMultipleChoiceField</strong> when i select an option and move to next step i am able to get selected option but when ever i move to the previous step and when submit the form it gives below exception</p>
  
  <blockquote>
    <p>if not queryset._prefetch_related_lookups:<br>
     AttributeError: 'NoneType' object has no attribute '_prefetch_related_lookups'</p>
  </blockquote>
  
  <pre><code># **form 1**
  class Form1(forms.Form):
      person = forms.ModelMultipleChoiceField(label=_("Student"), queryset=None, required=False)
  
  # **form 2**
  class Form2(forms.Form):
      classification = forms.ChoiceField(label=_("Classification"), choices=classification_appointment)
  
  
  # **view**
  class AddAppointment(SessionWizardView):
      template_name = "appointment/appointment_form_add_using_stepper.html"
  
      def get(self, request, *args, **kwargs):
          try:
              form = self.get_form()
              if form.prefix == "0":
                  form.fields["person"].queryset = Person.objects.filter(user_id=self.request.user.id,
                                                                         classification="student").order_by(
                      'first_name')
  return self.render(form)
          except KeyError:
              return super().get(request, *args, **kwargs)
  #**url**
  path('application_form/', AddAppointment.as_view([Form1, Form2])
  </code></pre>
  
  <p><strong>NOTE:
  requirements</strong>
  python 3.7
  Django==2.2.7
  django-formtools2.2</p>