Fix some races due to promises completing after we've switched rooms

[richvdh]

Add a few isMounted() checks to promise handlers so that we don't end up
throwing NPEs.

This should fix element-hq/element-web#589.

[kegsay]

This probably needs to be self?

[richvdh]

apparently. Evidently this was super-well-tested :(

[richvdh]

done

[kegsay]

LGTM aside from self/this thing.

It is worth noting that in their infinite wisdom, Facebook are planning on deprecating isMounted() because they deem it to be an anti-pattern. They argue that all async operations should be shot in the head at the moment of componentWillUnmount. This is easy to do for callbacks (unregister the listener!). This is not so easy with promises/coroutines. Their suggested "right" way is horrible : monkey-patch the Promise class to include a cancel() method. Cancelling promises sensibly remains an open problem, so the idea of doing this leaves a bad taste in my mouth. Either way, this is what they've decided to do.

This means that in a later version of React, this code will be broken. We can either pre-emptively catch this (and have our own isMounted() check with a boolean marked on componentWillMount and componentWillUnmount), do the promise monkey patch, or leave it as is. I'd opt for #1 or #3 with a comment explaining the problem.

[richvdh]

ugh. Tiresome.

[richvdh]

@kegsay: ptal

[kegsay]

LGTM