SendMessage cannot be called during Awake, CheckConsistency, or OnValidate prevents UI from showing

To repro:
1. Open project attached (or import uMMORPG project).
2. Press Play and you'll get errors in log.

Actual result: warnings in log
SendMessage cannot be called during Awake, CheckConsistency, or OnValidate
UnityEngine.UI.Slider:OnValidate()

Expected result: No warnings in log.

In 2017.1.0f3 works with no warnings.
In 2017.2.0a2 - b3 warnings appear.