Ok I think I got something - look at the on instance message in your error. It is validating a None response, not your payload. This is what I get if I enter an intentionally wrong payload:
>>> payload = "wrong"
>>> action._validate_action_result(payload)
...
Failed validating 'type' in schema:
{'properties': {'events': {'items': {'properties': {'event': {'type': 'string'}},
'type': 'object'},
'type': 'array'},
'responses': {'items': {'type': 'object'},
'type': 'array'}},
'type': 'object'}
On instance:
'wrong'
vs. in the original error
Failed validating 'type' in schema:
{'properties': {'events': {'items': {'properties': {'event': {'type': 'string'}},
'type': 'object'},
'type': 'array'},
'responses': {'items': {'type': 'object'},
'type': 'array'}},
'type': 'object'}
On instance:
None