Warn when second argument to render is null and document.body is null as well

[gaearon]

Too often I run into this:

 Invariant Violation: _registerComponent(...): Target container is not a DOM element.

when making a new project / example.

The reason is I'm passing document.body but React's script is in head and body doesn't exist yet.

This is very common problem as I observed for beginners, as they tend to put their scripts in head but React samples use document.body as mounting point.

The real problem here is the message: you wouldn't believe that document.body "is not a DOM element" and assume something else is wrong, without realizing it may be empty at the time.

I suggest we show a different, more specific warning when second argument is null and document.body is null. Something like "Target container is null. Are you trying to mount to document.body before it is ready?"

[syranide]

👍 I think many don't really understand what $(function() { ... }) does and do it just because their code didn't work otherwise, I like the "document.body warning"!

[sophiebits]

This sounds vaguely reasonable, but I think we're going to do #3211 instead so I'll close this.