Focus scoped to null element

I'm guessing this is probably an issue that's causing people problems...

There's a line of code in ModalPortal that has this:

``` javascript
focusManager.setupScopedFocus(this.node);
```

No where in `ModalPortal.js` do I see a place were `this.node` is actually set to something.

See https://github.com/reactjs/react-modal/blob/master/lib/components/ModalPortal.js#L80
