add return nil to compile

storage/mongodb.go

@@ -141,6 +141,9 @@ func (s MongoDBStorage) UpdateSubscription(eventid string, subscriber string, de
 			return er
 		}
 	}
+	return nil
+}
+
 func (s MongoDBStorage) UpdateEvent(event Event) error {
 	collection := s.Client.Database(s.DbName).Collection(s.Collections["events"])
 	if _, err := collection.ReplaceOne(context.TODO(), bson.M{"_id": event.ID}, event); err != nil {