It looks like a duplicate of old issue https://github.com/localstack/localstack/issues/78
But now it's reproducible for me with the latest version of LocalStack. 
stack_name = self._get_param('StackName')
        stack = self.cloudformation_backend.get_stack(stack_name)
        if not stack:
            raise ValidationError(stack_name,
                message='Unable to find stack "%s" in region %s' % (stack_name, aws_stack.get_region()))
        return cf_describe_stack_events_orig(self)
    cf_describe_stack_events_orig = responses.CloudFormationResponse.describe_stack_events
    responses.CloudFormationResponse.describe_stack_events = cf_describe_stack_events
def inject_stats_endpoint():
    """ Inject a simple /_stats endpoint into the moto server backend Web app. """
    
    from moto import server as moto_server
    def _get_stats():