fix observer null issue

[onmylife]

After several routing, I saw some element occur null exception on $nextTick.
So I added checking logic for observer null or not.
if ovserver is null, call estoryObserver().

[ymchun]

@onmylife actually it's not necessary to call destroyObserver() since the error is thrown by previous observer instance (because of $nextTick) which is already destroy.

[ymchun]

@Akryum could you please merge this PR? Thank you!

[onmylife]

@ymchun I agree. Destroy code is removed.

[zamarawka]

@Akryum could it be merged? I faced with this problem.

[KevinBon]

I'm experiencing the same issue on this example.

The bug is triggered at the initialisation and when using the pagination.

[simllll]

We just ran into the same issue, just saw this PR which is actually the same like #72 . What's the reason why this issue is not fixed yet?

[ymchun]

@simllll may be the author no longer maintaining this repo :(

[simllll]

looks like this, therefore I just created @hokify/vue-observe-visibility@0.5.0

• upgrades all deps (babel7,..etc)
• fixed this bug

see https://github.com/hokify/vue-observe-visibility/commits/master

[Akryum]

Thanks for the PR!