Fix component async warning

[nathan-isaac]

  console.error node_modules/react-dom/cjs/react-dom.development.js:545
    Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.
        in Thing (at App.js:6)
        in App (at App.test.js:7)

References

• https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html
• facebook/react#5465
• https://reactjs.org/blog/2015/12/16/ismounted-antipattern.html