It might be useful if you could share a zip of your .hype document; this would help reproduce the error and also better understand your setup. I'm going to assume you just need to call a function made within the Hype document from an HTML Widget element. The most basic way would be to call into a parent like:
parent.window.HYPE.documents["index"].functions().untitledFunction();
("index" may need to be changed in your case)
However, this probably won't work if your HTML Widget is on a different domain than the parent HTML page. If you intend both will be using the same domain name/web server, then you can probably upload your document and see if the error goes away.
If they will be on different domains then you'll need to use the postMessage method to communicate information between iframes. This is a bit more complex of code; there's a couple solutions I've seen in the past that could be adapted to work for you: