rasa: Slack : Bot doesnt respond at all : "TypeError: 'NoneType' object is not iterable". trace =>line 203, in _sanitize_user_message , for uid_to_remove in uids_to_remove:

Rasa version: Rasa: 1.10.14

Rasa SDK version (if used & relevant): NA

Rasa X version (if used & relevant): NA

Python version: Python 3.7.9

Operating system (windows, osx, …): Windows 10

Issue: I am testing a bot on slack after deploying it successfully in slack - I get : “TypeError: ‘NoneType’ object is not iterable” Please note that the same Bot works well with “rasa shell” The problem is ONLY seen when trying to test the bot in Slack.

Error (including full traceback):

Traceback (most recent call last): File “c:\users\smohana\anaconda3\envs\rasa_new\lib\site-packages\sanic\app.py”, line 976, in handle_request response = await response File “c:\users\smohana\anaconda3\envs\rasa_new\lib\site-packages\rasa\core\channels\slack.py”, line 384, in webhook user_message, metadata[“users”] File “c:\users\smohana\anaconda3\envs\rasa_new\lib\site-packages\rasa\core\channels\slack.py”, line 203, in _sanitize_user_message for uid_to_remove in uids_to_remove: TypeError: ‘NoneType’ object is not iterable`

Command or request that led to error: Just trying to interact with utter_greet in Slack

**Content of configuration file (config.yml)** (if relevant):
``
# Configuration for Rasa NLU.
# https://rasa.com/docs/rasa/nlu/components/
language: en
pipeline:
  - name: WhitespaceTokenizer
  - name: RegexFeaturizer
  - name: LexicalSyntacticFeaturizer
  - name: CountVectorsFeaturizer
  - name: CountVectorsFeaturizer
    analyzer: "char_wb"
    min_ngram: 1
    max_ngram: 4
  - name: DIETClassifier
    epochs: 150
  - name: EntitySynonymMapper
  - name: ResponseSelector
    epochs: 150


# Configuration for Rasa Core.
# https://rasa.com/docs/rasa/core/policies/
policies:
  - name: MemoizationPolicy
  - name: TEDPolicy
    max_history: 5
    epochs: 100
  - name: MappingPolicy

Content of domain file (domain.yml) (if relevant): `` NA