Set babel.

src/utils/Portal.js

@@ -0,0 +1,15 @@
+/*
+ * Copyright (C) 2019 CZ.NIC z.s.p.o. (http://www.nic.cz/)
+ *
+ * This is free software, licensed under the GNU General Public License v3.
+ * See /LICENSE for more information.
+ */
+
+import ReactDOM from 'react-dom';
+
+export default function Portal({containerId, children}) {
+    const container = document.getElementById(containerId);
+    if (container)
+        return ReactDOM.createPortal(children, container);
+    return null;
+}

src/utils/index.js

@@ -0,0 +1,3 @@
+import Portal from './Portal';
+
+export {Portal}