[bug 915836] Fix Add an Event failure when address not selected via Autocomplete

* Bugzilla URL: https://bugzilla.mozilla.org/show_bug.cgi?id=915836
* Next bug I'm going to tackle (https://bugzilla.mozilla.org/show_bug.cgi?id=927536) will add code paths / error message handlers. There is already a bug for that and I didn't want to turn this into a superbug.