Error when hidding keyboard and using Navigation extension

[jmsanzg]

When keyboard is being hidden, a call to navigation extension checkKeys() method is made but with an undefined key. That's the way to detect base.$keyboard doesn't exist and return instead of throwing an exception

[Mottie]

I would merge this, but there isn't a guarantee that undefined is really undefined. I tend to use typeof(key) === "undefined". I know that doesn't compress well.

If you make that change, I'll merge it when I have some more internet time.