code: def get_form_kwargs(self):
          kwargs = super().get_form_kwargs()
          
          if self.request.method == 'POST':
              data = copy(self.request.POST)
              i = 0
              while(data.get('%s-%s-id' % (
                  settings.FLAT_MENU_ITEMS_RELATED_NAME, i
              ))):
                  data['%s-%s-id' % (
                      settings.FLAT_MENU_ITEMS_RELATED_NAME, i
                  )] = None
                  i += 1
              kwargs.update({
                  'data': data,
                  'instance': self.model()
              })
          return kwargs
docs: When the form is posted, don't pass an instance to the form. It should
          create a new one out of the posted data. We also need to nullify any
          IDs posted for inline menu items, so that new instances of those are
          created too.