React-Router only one child

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

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

should be

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

jsfiddle / webpackbin