fix ParticipantRemove view’s get_queryset

cfp/views.py

@@ -6,7 +6,7 @@ from django.urls import reverse
 from django.utils.translation import ugettext_lazy as _
 from django.views.generic import DeleteView, FormView, TemplateView
 from django.contrib import messages
-from django.db.models import Q, Count
+from django.db.models import Q
 from django.views.generic import CreateView, DetailView, ListView, UpdateView
 from django.http import HttpResponse, Http404
 from django.utils import timezone
@@ -830,7 +830,7 @@ class ParticipantRemove(StaffRequiredMixin, OnSiteFormMixin, DeleteView):
     success_url = reverse_lazy('participant-list')
 
     def get_queryset(self):
-        return Participant.objects.annotate(talk_count=Count('talk')).filter(talk_count=0)
+        return Participant.objects.filter(talk__isnull=True)
 
 
 @staff_required