this change should be reverted (i.e., leave the try/catch). 

this is the **only** way to safely unsubscribe, as [NSHipster](http://nshipster.com/key-value-observing/) notes:
>Perhaps the most pronounced annoyance with KVO is how it gets you at the end. If you make a call to –`removeObserver:forKeyPath:context:` when the object is not registered as an observer (whether because it was already unregistered or not registered in the first place). The kicker is that _there's no built-in way to even check if an object is registered!_