Bad error message for .get(null)

[default0]

If you accidentally call an elements get method with null you get an error message telling you that you cannot call toString() on null, probably while restangular is trying to internally assemble the path based on the id you supposedly should have given it.

Restangular should instead catch this case up-front and tell you that the id parameter in the get function is required and cannot be null.

[daviesgeek]

Sounds like a great idea! Can you submit a PR with those changes?