add missing label_from_instance for SafeOrderPositionMultipleChoiceField

src/pretix/control/forms/organizer.py

@@ -254,6 +254,9 @@ class SafeOrderPositionMultipleChoiceField(forms.ModelMultipleChoiceField):
         queryset = queryset.model.all.none()
         super().__init__(queryset, **kwargs)
 
+    def label_from_instance(self, op):
+        return f'{op.order.code}-{op.positionid} ({str(op.item) + ((" - " + str(op.variation)) if op.variation else "")})'
+
 
 class EventMetaPropertyForm(I18nModelForm):
     class Meta: