fix: Allow mutable object refs to be used for FocusTrap child

[scottrippey]

What

• Fixes this.node.current is null. #50
• Allows mutable object refs, created by React.createRef(), to be used on the first child of the <FocusTrap> component

Testing

Here is a simple sample component that demonstrates the problem and the fix.
The problem is more prevalent when using the newer React Hooks, since most refs will be mutable object refs instead of callbacks.

import React from 'react';
import FocusTrap from 'focus-trap-react';

class Example extends React.PureComponent {
  constructor(props) {
    super(props);
    this.ref = React.createRef();
  }
  
  componentDidMount() {
    if (!this.ref.current) {
      throw new Error('ref.current should be a reference to the button!');
    }
  }

  render() {
    return (
      <FocusTrap>
        <button ref={this.ref}>
          Button
        </button>
      </FocusTrap>
    );
  }
}

[stefcameron]

@scottrippey Thanks for this trivial fix! Looks good to me. I can confirm it fixes my issue with functional components as well.

@davidtheclark Any chance we could get this merged and published?

[stefcameron]

@scottrippey Thanks for this PR! Merging...