Storybook: 「Target container is not a DOM element」error happened when using ReactDOM.createPortal method

Describe the bug
I'd like to add a story of my Modal component which is made by ReactDOM.createPortal .
the modal container is <div id="modal-root"></div> added by me in index.html .
but when I write story like this:

storiesOf('atoms/modal', module)
  .addDecorator(withKnobs)
  .add('Modal', () => (
    <Modal
      isOpen={true}
      onRequestClose={action('onRequestClose')}
    >
      <div>modal</div>
    </Modal>
  ))

「Target container is not a DOM element」error happened.

I think it's because there is no modal-root div in Storybook UI?
How can I fix it?

To Reproduce
see above

Screenshots

Code snippets
this is my Modal code for short:

const modalRoot = document.getElementById('modal-root')

class Modal extends React.Component {
  constructor(props) {
    super(props)
    this.el = document.createElement('div')
  }

  componentDidMount() {
    modalRoot.appendChild(this.el)
  }

  componentWillUnmount() {
    modalRoot.removeChild(this.el)
  }

  handleClickBackDrop = event => {
   this.props.onRequestClose(event)
  }

  render() {
    const { isOpen, children } = this.props

    if (!isOpen) {
      return null
    }

    const modalUI = (
      <BackDropWrapper
        onClick={this.handleClickBackDrop}
      >
        <ModalWrapper>
          {children}
        </ModalWrapper>
      </BackDropWrapper>
    )

    return ReactDOM.createPortal(modalUI, modalRoot)
  }
}

System:

• Framework: react
• Addons: addon-knobs
• Version: 4.0.0-alpha.3