Use getter syntax to unbox observable

`state.frames` is an observ-hash or something alike

This means `state.frames[frameId]` returns an observable not
  a value.

If we set `currentFrame` to the observable then `currentFrame`
  is an observable containing an observable containing the value.

We need to unpack `state.frames[frameId]()` to ensure that
  `currentFrame` is only an observable of a value instead of a
  nested observable