Steps to reproduce:

1. Go to cache creation wizard and choose normal media-type: application/x-protostream;
2. On the next page choose 'indexing' capability and choose the entities class to index;
3. Go back to previous 'Basic options page' and choose e.g. 'application/json'
4. Go to 'Review configuration' page directly from left hand menu
5. On the page in the code editor you will see that cache config contains media-type 'text/plain' but also contains the indexing part.
6. By clicking the `Create` button the error popup is shown (see the screenshot), but the cache is still created as if you will go back to Data Container page, you will see it with status "Failed".