TypeError: Cannot read property 'firstChild' of null

Getting an error coming from line 37 of source:

`<div ref={r => this.scrollingElement = r.firstChild}>`

It seems like the ref is null.

Here is my implementation of it:

```
<ScrollLock>
       <div>
           <ul>
                { /* more stuff in here */ }
            </ul>
       </div>
</ScrollLock>
```