renderWhenReady errors with 'can't access property "getBoundingClientRect"'

I have what I think is a very straightforward invocation:

```js
webamp = new Webamp({
initialTracks: [{
url: "/audio/0 - Paint Exchange.flac",
}],
enableHotkeys: true,
volume: 75
});

webamp.renderWhenReady().then(() => {
console.log("rendered webamp!");
}).catch(error => {
alert(error.message)
});
```

...and unless I pass a DOM element to `renderWhenReady`, it fails with an error saying that it can't access `getBoundingClientRect`.

It seems like it's trying to access properties of the DOM element that I'm not passing.

I'm sure I'm doing something silly, and the problem is right in front of me. :-) But in the unlikely event that it's a browser or library compatibility issue, I figured I'd document it here until I figure it out.

Start by reproducing the supplied Webamp constructor and renderWhenReady() call, then trace how the method handles its DOM-element argument before accessing getBoundingClientRect. Confirm whether the no-argument call is supported and add a regression check for the shown invocation; done means the behavior is corrected or clearly documented.

Written by the indexing model from the issue text.