Tax rule creation: Fix crash on invalid form submission

src/pretix/control/views/event.py

@@ -1104,6 +1104,7 @@ class TaxCreate(EventSettingsViewMixin, EventPermissionRequiredMixin, CreateView
         }
 
     def post(self, request, *args, **kwargs):
+        self.object = None
         form = self.get_form()
         if form.is_valid() and self.formset.is_valid():
             return self.form_valid(form)