We need the check in `notify_should_kill` in case it is called when the device input thread is destroyed but we're not actually ready to shutdown (in the case of the phone being unplugged for example). But then we also need to check for `should_kill` here: https://github.com/mozilla/r2d2b2g/pull/752/files#L2R240 because we need to `return null` if `should_kill` is true. I'll make `notify_should_kill` return `should_kill`; I think that's the best solution.