Rasa X: error when launching

Rasa version: 2.6.0

Rasa SDK version: 2.6.0

Rasa X version: 0.39.3

Python version: 3.8.8

Operating system: linux (ubuntu 18.04)

Hi community,

When launching rasa x on local server with custom action_session_start, I got the following error:

[2021-05-11 14:54:04 +0200] [39781] [ERROR] Exception occurred while handling uri: 'http://localhost:5005/conversations/0e373bfa125d4c619095e8ffc3403a78/tracker?include_events=ALL'
Traceback (most recent call last):
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/sanic/app.py", line 938, in handle_request
    response = await response
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/server.py", line 261, in decorated
    result = await result
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/server.py", line 718, in retrieve_tracker
    tracker = await app.agent.create_processor().fetch_tracker_with_initial_session(
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/core/processor.py", line 266, in fetch_tracker_with_initial_session
    await self._update_tracker_session(tracker, output_channel, metadata)
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/core/processor.py", line 211, in _update_tracker_session
    await self._run_action(
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/core/processor.py", line 798, in _run_action
    await self.execute_side_effects(events, tracker, output_channel)
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/core/processor.py", line 699, in execute_side_effects
    await self._send_bot_messages(events, tracker, output_channel)
  File "/home/matthieu/anaconda3/envs/rasa-os-2.6/lib/python3.8/site-packages/rasa/core/processor.py", line 715, in _send_bot_messages
    await output_channel.send_response(tracker.sender_id, e.message())
AttributeError: 'NoneType' object has no attribute 'send_response'

Would anyone have an advice?