selection: ensure that `this.range` is defined

In `getContaining()` and `placeMarkers()` functions.

Prevents uncaught errors as described in #204.
I agree that it's bad, but I want to solve it as per https://github.com/guardian/scribe/pull/231/files#r14988983.

I would like to understand why/how/when you're creating a selection object without the editor focused. If you're creating a selection without the editor focused, it might be that you're doing something wrong on your end, or it might not. I just want to gather an understanding before I merge this workaround..