Prevent the renderer from hanging if the main model panics during its init

[timmattison]

This should fix #904

The issue arises if the main model panics during init. If this happens it tries to send a signal via r.done. But nothing is reading r.done until go r.listen() is called in r.start().

In r.start() the r.ticker value is set up, then r.once is initialized, and then finally go r.listen() is called. Since r.ticker is only assigned a value in this one place we can be reasonably certain that if it is non-nil that go r.listen() has been called and it is monitoring the r.done channel.

An alternate solution would be to add a new variable to indicate that the system has entered r.listen(). I like that solution better, but I wasn't sure if it makes sense to add another variable for this edge case.

[meowgorithm]

Hi! Does #926 fix this? If so, would prefer that one as it’s a simpler change.

[timmattison]

It looks like it does but I haven't tested it. I prefer that one too if it works.

[meowgorithm]

Just checked and it appears to. Speaking of which, thank you for the very well documented issue; made it very easy to vet.

[maaslalani]

Closing this PR as it seems that #926 solves the issue.

Please feel free to re-open this PR if #926 doesn't solve #904 / #924 after it's merged.