Same problem here, same error in log. If I keep hitting restart, after a few errors it picks up and restarts. But it takes a very long time before I can call the UI again.
Also get this in the log after restart:
2018-12-23 15:03:28 WARNING (Recorder) [homeassistant.components.recorder] Ended unfinished session (id=4 from 2018-12-22 17:30:55.496863)
Don’t know if it’s related.