old_contents: from comics.sets.models import UserSet
  
  def user_set(request):
      try:
          user_set = UserSet.objects.get(user=request.user)
          return {
              'user_set': user_set,
              'user_set_comics': user_set.comics.all(),
          }
      except UserSet.DoesNotExist:
          return {}
  
new_contents: def user_set(request):
      if hasattr(request, 'user_set'):
          return {
              'user_set': request.user_set,
              'user_set_comics': request.user_set.comics.all(),
          }
      else:
          return {}
  
subject: Use request.user_set in context preprocessor