Reproduction steps:
1. Open project attached (New Unity Project 15)
2. Press "Menu/Test/Test"

Expected result: Prefab will be created correctly.
Actual result: TypeLoadException error appears.
Error message: TypeLoadException: Could not load type 'UnityEngine.AvatarMask' from assembly 'UnityEditor, Version=0.0.0.0, Culture=neutral,
PublicKeyToken=null'.

Notes: Updater does not appear properly. Sometimes you need to send it to background or just reimport everything to make it appear.

Reproduced on: 5.6.0f3, 5.6.1p1, 2017.1.0b6.
Works fine on 5.5.3p3.