To reproduce:

1. Create an Index Threshold alert with an associated action (I used Slack)
2. Delete the connector for the associated action.
3. Create a test user with role that has "All" access to "Built-In Alerts" and "Read" access to "Actions"
4. Log in as the test user and navigate to "Alerts and Actions"
5. Select the alert created in Step 1 and click "Edit"
6. Notice that the action associated with the alert has a message "No Slack connectors" but your user doesn't have the privileges to configure a connector.
7. Notice that the "Save" button is enabled but when you try to Save without removing the broken action, you will receive a 400 error message.