[Security Solution][Detections] Cannot add an action to an alert

[cnasikas]

Kibana crashes when you try to add an action to a detection rule.

Steps to reproduce it:

1. Go to Detections -> Manage detection rules -> Create new rule.
2. Fill all needed fields in the first three steps.
3. At step 4 (rule actions) select the frequency of action.
4. Select an action type. If you don't have a connector create one.
5. You should see the following error: TypeError: Cannot read property 'map' of undefined

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)