Fix of flow type error regarding getElementById
Fix of flow type error regarding getElementById

```
Error: src/index.js:45
 45:     (document.getElementById('root'))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ null. This type is incompatible with the expected param type of
 17:     container: Element,
                    ^^^^^^^ Element. See lib: /tmp/flow/flowlib_2cca3f39/react-dom.js:17
```

mareklibra/userportal,oVirt/ovirt-web-ui,mareklibra/userportal,mareklibra/userportal,oVirt/ovirt-web-ui,mareklibra/userportal,oVirt/ovirt-web-ui