React-Router only one child

You have to wrap your Route's in a <div>(or a <Switch>).

render((
  <Router>
    <Route exact  path="/" component={BaseLayer} />
    <Route path="/editor" component={App} store={store} />
  </Router>
), document.querySelector('#app'));

should be

render((
  <Router>
    <div>
       <Route exact  path="/" component={BaseLayer} />
       <Route path="/editor" component={App} store={store} />
    </div>
  </Router>
), document.querySelector('#app'));

jsfiddle / webpackbin

This is an API change in react-router 4.x. Recommended approach is to wrap Routes in a Switch: https://github.com/ReactTraining/react-router/issues/4131#issuecomment-274171357

Quoting:

Convert

<Router>
  <Route ...>
  <Route ...>
</Router>

to

<Router>
  <Switch>
    <Route ...>
    <Route ...>
  </Switch>
</Router>

You will, of course, need to add Switch to your imports:

import { Switch, Router, Route } from 'react-router'