react: "Missing React element for debugID" warning when triggering a render in componentWillMount

Do you want to request a feature or report a bug? Bug

What is the current behavior? As reported by @adamryvola in https://github.com/facebook/react/pull/6869#issuecomment-230349514, React is displaying a warning Warning: ReactComponentTreeDevtool: Missing React element for debugID 2 when building stack when another component is rendered in componentWillMount

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/reactjs/69z2wepo/).

import React, {Component} from 'react';
import ReactDOM from 'react-dom';

const container = document.getElementById('container');
const menuContainer = document.getElementById('menu-container');

export default class App extends Component {

  componentWillMount() {
    ReactDOM.render(<div />, menuContainer);  // <= Trigger the warning
  }

  render() {
    return (
      <div>
        <div>App</div>
      </div>
    );
  }
}

ReactDOM.render(<App />, container);

JSFiddle: https://jsfiddle.net/0n3a6vp9/

What is the expected behavior? According to @gaearon this warning is “against internal bugs in React, it’s not meant for users” (source: https://github.com/facebook/react/pull/6869#issuecomment-230272365) so this warning should not be triggered.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React? React 15.2.0, no issue with React 0.14.7