null scrollbar fix for componentWillUnmount
null scrollbar fix for componentWillUnmount

In some cases, `componentWillUnmount` is called before `componentDidMount`, which leaves `this.scrollbar` as null and causes a `Cannot read property 'destroy' of null` error. [See this issue in React](https://github.com/facebook/react/issues/5719)
noeldelgado/react-gemini-scrollbar,noeldelgado/react-gemini-scrollbar