Add support / better error logging for functional stateless children components

[Zhang]

Portal will render child stateless children components
const PopoverMenu = ({...props}) => {
, but will trigger this error:

The fix is to write your components like:
const PopoverMenu = ({...props}) => {

Unfortunately, this is very unintuitive to debug. It would be helpful to have better error messaging or general support around this issue

[tajo]

Your sample code for problem and solution is identical?

[tajo]

And yes, the support for functional components is iffy currently.

[Zhang]

Oops - wrong paste, I had meant to write:

export class PopoverList extends Component {

[tajo]

Hey, please check the new major version of react-portal: #157

It's React v16 only since its uses the new official Portal API. There is the first beta released and I would like to get your feedback. I don't have bandwidth to maintain v3 which is very different and full of hacks.

I am happy to discuss this with v4 in mind. If this is still a problem, please re-open/update the issue. Thanks.