Entering Editor Play mode with this character's Blend Tree preview in the Inspector causes an error in the console and default avatar to be displayed instead of expected imported character.

repro:
-import attached BlendError.unitypackage
-open errorBug scene
-open Animator window
-select mangagirl in the scene
-double-click the Blend Tree state to enter it
-click the Blend Tree node to display the Blend Preview in the Inspector
-click the Editor Play button (not preview play)
bug: error in the console and default avatar in the preview window
(see attached gif)

error:
get_skinIndex is not allowed to be called from a MonoBehaviour constructor, call it in Awake or Start instead. Called from script 'BlendTreeInspector' on game object ''.
See "Script Serialization" page in the Unity Manual for further details.
UnityEditor.BlendTreeInspector:.cctor()

Un-selecting, then re-selecting Blend Tree forces an update of the preview window and fixes the issue. Expected character is displayed again.

this is a regression from 5.3
no error in 5.3.2p2 (dec88fd2a56c)
repro in...
Version 5.4.0b6 (2c3d6b93e789)
Branch: 5.4/staging
... and animation/dev