Hmm, not sure then. There are only two references to _showFeedback in Adapt itself, in app.js and questionView.js - and I can't see how either of those would trigger this error.

Is there anything custom/odd about the project i.e. any non-core components or is it an old version of the framework?

Using $ grunt dev to compile your code will leave 'source mapping' on which might give more detail about where in the code the problem is