Slack Notifier failed to unmarshal templating JSON

I have been following these instructions on the [GCP documentation page](https://cloud.google.com/build/docs/configuring-notifications/configure-slack). I have done nothing custom beyond substituting variables.

However, it is not working and I see tons of error messages in the cloud run logs saying:
```
1 notifiers.go:549] failed to run SendNotification: failed to write Slack message: failed to unmarshal templating JSON: unexpected end of JSON input"
```

There is a somewhat similar [closed issue here ](https://github.com/GoogleCloudPlatform/cloud-build-notifiers/issues/105)- and the solution was to pin an older version. If thats the case, what version should it be?

Also, how would I set the retry limit invocations to the cloud run service from pubsub? I think would have racked up quite the bill if I had not been looking.