Warning: Can't perform a React state update on an unmounted component logged when visiting most pages

[toolness]

This is coming from NotRegisteredPage, which is particularly odd because it's being logged on pages that are registered. It seems like something's causing the component to be mounted momentarily; that component starts a network request on mount and a setState on request completion, but by that point the component has already been unmounted, hence the warning.

This was likely introduced by #346.