Unregister ResizeObserver for map container

[chrismayer]

This unregisters the ResizeObserver for the map's container element, when the map is unmounted/destroyed.

Besides having a better cleanup this also removes the null pointer access within the ResizeObserver handler in the hot-reload of the dev- setup, when then WguApp.vue is changed.

[fschmenger]

Nice catch, I have currently no time to test but totally makes sense.

For paranoia reasons I suggest to wrap up the disconnect with an
if (this.resizeObserver)
just in case container is undefined on registration.

[chrismayer]

Thanks for your review @fschmenger!

I added the check if (this.resizeObserver), not just for paranoia 😉 - it was necessary to make the tests green. Here we exactly have the situation, that no container exists. Thanks for the hint!