Glad you like the Visualization API...
I don't think your error message is related to your problem directly, but it is a symptom that something is not set up correctly. I am no expert in this, but I think you need to look at your proxy configuration. The actual call (qrs/extension/schema) is used to get the list of extensions, so it has nothing to do with selections and it seems like you are using the built-in charts. I assume you are running on an external web server. Does the same mashup work if you run it with Qlik Sense server the files??