assembly create: ensure conference is set early

src/backoffice/forms.py

@@ -83,6 +83,9 @@ class AssemblyCreateForm(forms.ModelForm):
         if (slug := self.cleaned_data.get('slug')) and Assembly.objects.filter(conference=self._conference, slug=slug).exists():
             self.add_error('slug', _('Assembly__slug__alreadyexists'))
 
+        # ensure Assembly's conference is set before saving
+        self.instance.conference = self._conference
+
 
 class AssemblyEditForm(TranslatedFieldsForm):
     class Meta: