title: current is always null when using React.createRef
body: <p>I was trying to do <a href="https://stackoverflow.com/questions/43817118/how-to-get-the-width-of-a-react-element">this</a>.</p>
  
  <p>I must be missing something, but I don't understand why <code>current</code> is always <code>null</code> in this example.</p>
  
  <pre><code>class App extends React.PureComponent {
    constructor(props) {
      super(props);
      this.test = React.createRef();
    }
    render() {
      return &lt;div className="App"&gt;current value : {this.test.current + ""}&lt;/div&gt;;
    }
  }
  </code></pre>
  
  <p>You can check my test case <a href="https://codesandbox.io/s/q4l6nzloyq" rel="noreferrer">here</a></p>