https://jsfiddle.net/3apqnhmw/

```jsx
class Foo extends React.Component {
  render() {
     return <div>Foo</div>
  }
}

class App extends React.Component {
  render() {
    return Foo
  }
}
```

When `App` returns `Foo` instead of `<Foo />` accidentally, Fiber does not catch this.
'inner 1 render',
            'inner 2 render',
            'ref 1 got null',
            'ref 2 got null',
          ]
        : [
            
            'ref 1 got null',
            'inner 1 render',
            'ref 2 got null',
            'inner 2 render',
          ]),
      'inner 1 componentDidUpdate',
      'ref 1 got instance 1',
      'inner 2 componentDidUpdate',
      'ref 2 got instance 2',
      'outer componentDidUpdate',
      'start unmount',
      'outer componentWillUnmount',
      'ref 1 got null',
      'inner 1 componentWillUnmount',
      'ref 2 got null',
      'inner 2 componentWillUnmount',
    ]);
    
  });
  it('fires the callback after a component is rendered', () => {
    var callback = jest.fn();
    var container = document.createElement('div');
    ReactDOM.render(<div />, container, callback);