This assertion fails sometimes when the Display settings is first loaded.

polymer-mini-extracted.js:2120 Uncaught Error: Assertion failed: Pref is missing: ash.night_light.custom_start_time
    at assert (crisper.js:20)
    at HTMLElement.getPref (crisper.js:361)
    at HTMLElement.customTimesChanged_ (crisper.js:476)
    at HTMLElement.<anonymous> (crisper.js:476)
    at polymer-extracted.js:1288
    at Object._atEndOfMicrotask (polymer-mini-extracted.js:2114)
    at MutationObserver.window.MutationObserver.observe.characterData (polymer-mini-extracted.js:2129)

I believe it's the async() call here: https://cs.chromium.org/chromium/src/chrome/browser/resources/settings/device_page/night_light_slider.js?q=night_light_slider&sq=package:chromium&l=84

Not sure why that is happening. It doesn't seem to be affecting anything, and moving the start slider knob works as expected. Is it a timing issue?