In my case the button and the modal is a single logical unit.
Splitting it into two components and share common state (ie flagShowModal) across
application seemis not a good idea.
I've found a solution that is called Portal (or Gateway) (http://stackoverflow.com/questions/28802179/how-to-create-a-react-modalwhich-is-append-to-body-with-transitions).
The idea, is that the Portal nothing renders in its own "render" function, but renders
child elements in a different place via ReactDOM.render on
componentDidUpdate event.