TypeError :CIMultiDict keys error when integrating Rasa with Slack

Rasa version : 1.2.2.

Rasa X version (if used & relevant): Not Used

Python version : 3.6.8

Operating system (windows, osx, …): Ubuntu 18.04

Issue : I have integrated Slack with Rasa. It shows verified in the Events Subscription page on Slack. But on sending a message to the channel/app on Slack, the following error is encountered.

Error (including full traceback) :

Exception occurred while handling uri: 'http://c23c2a14.ngrok.io/webhooks/slack/webhook'
Traceback (most recent call last):
  File "/home/rajat-mehta/.local/lib/python3.6/site-packages/sanic/app.py", line 917, in handle_request
    response = await response
  File "/home/rajat-mehta/.local/lib/python3.6/site-packages/rasa/core/channels/slack.py", line 329, in webhook
  File "/home/rajat-mehta/.local/lib/python3.6/site-packages/rasa/core/channels/slack.py", line 267, in process_message
    retry_reason = request.headers.get(self.retry_reason_header)
  File "multidict/_multidict.pyx", line 69, in multidict._multidict._Base.get
  File "multidict/_multidict.pyx", line 52, in multidict._multidict._Base._getone
TypeError: CIMultiDict keys should be either str or subclasses of str

slack: slack_token: “xxxxxxxxxxxxx” slack_channel: “@name” slack_retry_reason_header: “x-slack-retry-reason” #Slack HTTP header name indicating reason that slack send retry request. This configuration is optional. slack_retry_number_header: “x-slack-retry-num” #Slack HTTP header name indicating the attempt number. This configuration is optional. errors_ignore_retry: None #Any error codes given by Slack included in this list will be ignored. Error codes are listed here <https://api.slack.com/events-api#errors>_.

rasa: url: “http://localhost:5002/api

@Profile - Juste - Rasa Community Forum Please help.