splunkd.log Warning Logs - Attachment Must be Alert_Link or Message

[Justangatang]

Every time the Slack Notification Alerts app is called it provides a warning in the splunkd.log file. This seems to occur when the 'Attachment' field in the alert output is set to 'None'

Warning Example:
INFO sendmodalert [416 AlertNotifierWorker-0] - action=slack STDERR - Running python 3
WARN sendmodalert [416 AlertNotifierWorker-0] - action=slack STDERR - Validation warning: Parameter attachment must be ether "alert_link" or "message"
INFO sendmodalert [416 AlertNotifierWorker-0] - action=slack STDERR - Using configured webhook URL:###

Steps to reproduce the behavior:

1. Create an alert
2. Set attachment to 'None'
3. Have the alert generate and send to Slack

Splunk Version 8.2.2
Slack Notification Alerts App v2.2.0

I've also seen this same issue in the Splunk community forum when looking up the message:
https://community.splunk.com/t5/All-Apps-and-Add-ons/Attachment-must-be-ether-alert-link-or-message/m-p/477109