Send Test Notification via DM fails

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

1. Create a new Issue Alert
2. Set action to Slack notification
3. Add Slack username & user ID
4. Click `Send Test Notification`

### Expected Result

Receive Dummy Event via DM

### Actual Result

After saving the alert, the notification correctly sends an event via DM. So it seems to work but "Send test notification" is broken.

### Product Area

Alerts

### Link

_No response_

### DSN

_No response_

### Version

_No response_