check if observer is not null

[simllll]

fixes cannot read property of 'observe' of null we get from time to time. The handle is somehow destroyed before nextTick is called, and then it's null which causes this error. Simple check if observer is still defined before doing "observe()" fixes it.

[Akryum]

Thanks for the PR! It was already implemented in #39 though.

[simllll]

Great, thanks for merging it @Akryum .

two followups:
1.) can you also update the dependency of "vue-virtual-scroller" .. this was the reason why I came across the bug at all it works anyway, package dependency is not pinned to a specific version :)
2.) I did two forks of
vue-observe-visibility & vue-virtual-scroller.
besides this little bug, I updated to babel@7 and did a complete library update. Feel free to adapt / copy changes over to this rep.
https://github.com/hokify/vue-observe-visibility
https://github.com/hokify/vue-virtual-scroller

all the best,
and thanks for you work!
Simon

[simllll]

my fault, the vue-virtual-scorller is built, and therefore the dependency needs to be updated / rebuild to get into it. I will create a ticket, would be great if you can run it through ;) thanks