fix a 500 error when creating/editing notification templates

see: https://github.com/ansible/awx/issues/5028

awx/api/serializers.py

@@ -4406,6 +4406,8 @@ class NotificationTemplateSerializer(BaseSerializer):
             for event in messages:
                 if not messages[event]:
                     continue
+                if not isinstance(messages[event], dict):
+                    continue
                 body = messages[event].get('body', {})
                 if body:
                     try: