Failed to get correct selectedDocument
## What I have done

I open a new sketch document which named untitled by default and open my plugin(created by sketch-module-web-view), I can do something inside the sketch.
Then I create a new sketch document(command + N) which named untitled 243(may be), and when I do something in the plugin the effect always show in the first document window.

## What I expect

I hope everything I do in plugin will show in the current document window which I have focused latest.

## Others

I find that the `sketch-dev-tool` plugin created by you knows which document is selected and can log each document objectID in playground tab(`console.log(context.api().selectedDocument.selectedPage.id)`). And when I swipe down with four fingers to show App Exposé, I can see the dev tool panel. Is there any config I might have missed to toggle between document window? (I have browse the code of `sketch-dev-tool` but failed to figure it out).

Thanks for your help!