Unmounting of Callout Elements doesn't work

[nikischin]

I just figured out the unmounting of the callout elements implemented with #55 doesn't work.

I implemented the new feature into my app and whenever you navigate to some other view all the annotations overlay the screen. I first thought it would be something related to React Router having issues with Portals but seems like a rather general issue to me.

I will further investigate these days and probably also submit a PR to fix, though wanted to open this bug already if someone comes up with the same issue.