Maintenance mode broken w/ Lemonsync

[tomcornall]

https://sentry.io/lemonstand/lemonstand/issues/227959948/

LemonStand\Exception\SystemErrorException: () Fatal error: Call to a member function pages() on a non-object
  File "srv/www/lemonstand2/releases/20170321200632/modules/cms/Cms.php", line 505, in LemonStand\{closure}
    ->pages()
  File "[Anonymous function]", line 0
    LemonStand\Application->LemonStand\{closure}

[tomcornall]

Saving maintenance mode sets the system_settings "maintenance theme" to whichever theme is currently editing at the time.

If you delete a theme and don't re-save maintenance mode, the maintenance theme is still referencing the deleted theme.