Catches exception thrown by Ember 2.0 when no item is selected #91

[ninoreiter]

Ember 2.0 changed the behavior of Ember.get(obj, keyName). If obj is undefined or null, older versions of Ember return undefined. With the changes of Ember 2.0 this behavior changed and an exception is been thrown. This exception can be reproduced when no item is selected (selection = null).
The changes restore the old behavior of Ember.get by wrapping it in the method _getWithExceptionHandling.

See issue #91

[miguelcobain]

@ninoraubaum I think that testing obj instead would be more idiomatic. Something like:

_get(obj, keyName) {
  if (obj) { return get(obj, keyName); }
  else { return undefined; }
}

Do you agree, and if so, would you mind updating the PR?
Thanks a lot.