Integration error - Invalid JSON Response Received

Hello.

My addon activates just fine, and even gets the event details from the API; however, regardless of what I return (nothing, JSON, a full HTML page, or pieces of HTML) I get the following error in the modal window/iframe:

Invalid Response Received
Invalid JSON Response Received

I use Self hosted web service function and also try external integration too. Can you describe more about what kind JSON format need to return? can you give an example?

Thanks