Fix: an exception when Client.stop_listening is called but the future is None

[moha-abdi]

This is a simple change, you can reproduce the issue by creatting a Client.register_next_step_handler and then calling the stop_listening method, this will find the listener but the future will be None as register_next_step_handler doesn't use future and it will rise this error: 'NoneType' object has no attribute 'done'