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.
render() {
        return (
          <div>
            <Inner
              id={1}
              ref={c => {
                log.push(`ref 1 got ${c ? `instance ${c.props.id}` : 'null'}`);
              }}
            />
            <Inner
              id={2}
              ref={c => {
                log.push(`ref 2 got ${c ? `instance ${c.props.id}` : 'null'}`);
              }}
            />
          </div>
        );
      }
      componentDidMount() {
        log.push('outer componentDidMount');
      }
      componentDidUpdate() {