Support popup for react v16

[stenin-nikita]

• react version: 16.2.0
• react-dom version: 16.2.0
• bem-react-components version: master

Description:

When popup is rendered, the value of this._domNode is null. An error occurs here.

Uncaught TypeError: Cannot read property 'offsetWidth' of null (Popup_target.js:105)

This is most likely due to the fact that the render method in Portal-Popup returns null. Removing the method should solve the problem. Alternatively, you can use ReactDom.createPortal for react 16.