fix return value

Author: nxs7

py/selenium/webdriver/remote/websocket_connection.py

@@ -79,7 +79,7 @@ def execute(self, command):
             return self._deserialize_result(result, command)
 
     def add_callback(self, event, callback):
-        self.add_callback_with_sequence(event, lambda _, x: callback(x))
+        return self.add_callback_with_sequence(event, lambda _, x: callback(x))
 
     def add_callback_with_sequence(self, event, callback):
         event_name = event.event_class