old_contents: import React, { Component, PropTypes } from 'react'
  import Navbar from '../containers/navbar/Navbar'
  
  export default class App extends Component {
    static propTypes = {
      children: PropTypes.element.isRequired,
    }
  
    render() {
      return (
        <div ref="app">
          <Navbar />
          {this.props.children}
        </div>
      )
    }
  }
  
new_contents: import React, { Component, PropTypes } from 'react'
  import Navbar from '../containers/navbar/Navbar'
  
  export default class App extends Component {
    static propTypes = {
      children: PropTypes.element.isRequired,
    }
  
    render() {
      return (
        <div ref={(app) => { this.app = app }}>
          <Navbar />
          {this.props.children}
        </div>
      )
    }
  }
  
subject: update ref