set app element so the body is not automatically aria-hidden by the react modal default behavior

src/index.jsx

 import React from 'react';
 import ReactDOM from 'react-dom';
+import Modal from 'react-modal';
 
 import AppStateHOC from './lib/app-state-hoc.jsx';
 import GUI from './containers/gui.jsx';
@@ -13,4 +14,6 @@ const appTarget = document.createElement('div');
 appTarget.className = styles.app;
 document.body.appendChild(appTarget);
 
+Modal.setAppElement(appTarget);
+
 ReactDOM.render(<App />, appTarget);