Unmounting the mount target may crash the app if the node doesn't exist

[erquhart]

First off, thanks for this lib!! We've been using it since the beginning in Netlify CMS, really glad it's here :)

So we ran into an issue in the latest version of Firefox Stable, 56.0.2, where attempting to unmount the mount target in Frame's componentWillUnmount handler errors out because the target isn't there. I haven't traced this all the way back, but it seems like some kind of race condition.

I also have a feeling that this isn't a new bug, but that perhaps it was just less obvious before React 16 made every error a showstopper, and was maybe even recoverable when the parent of Frame updated after a failure.

Anyway, just a heads up, PR forthcoming. If it's at all possible to get a quick release I'd be super grateful. Thanks!!

[erquhart]

@ryanseddon this was regressed by #94 in 3.0.