Uncaught TypeError: Cannot read property 'x' of null

if you call `chart.flow(…)` twice immediately or after a short period of time you get the following error:

> Uncaught TypeError: Cannot read property 'x' of null

It's thrown in `c3.js` at line `6227`:

``` javascript
} else if (flow.orgDataCount === 1 || flowStart.x === flowEnd.x) {
```

because `flowStart` is `null`.

Here's a CodePen illustrating the problem: http://codepen.io/mmiszy/pen/xGPYEj
The exception might occur even if the second call to `chart.flow(…)` happens after 300ms (maybe longer?)