workaround for ember v3.1 compatability

[toranb]

As of ember v3.1 the Proxy used by core object runs a strict assertion that in turns throws the error

Since Ember 3.1, this is usually fine as you no longer need to use `.get()` to access computed properties. However, in this case, the object in question is a special kind of Ember object (a proxy). Therefore, it is still necessary to use `.get('X')` in this case

Robert Jackson was able to further identify this after I submitted an issue a few days back ;)

While this PR isn't anything I'm extremely happy about, it does offer a workaround for anyone upgrading to ember v3.1. I'm planning to submit a more correct fix to ember core itself based on the feedback

[toranb]

emberjs/ember.js#16560