When we create/update a entity, it gives error

Steps to Reproduce:
1. Go to the module configuration.
2. Select any entity(content).
3. Then select the notification operation for that entity. And Save the configuration.
4. Create a selected entity(node). And check

Actual Result:
1. When we create/update any entity type, then it gives us an error.
2. When Custom slack Message is not configured, then default message notification not working.

Expected Result:
1. When we create/update any entity type, then it should not throws any error.
2. When Custom Slack Message for the entity is not configured, then the user should receive the default message.