How to reproduce:

1. Open attached project
2. Open scene scene.unity
3. Open the Animator window
4. Go down into the Blend Tree layer
5. Select the Blend Tree
6. Play the scene
- Note the error in the console:
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()
- Reproduced in Version 5.4.0b6 (2c3d6b93e789), Version 5.4.0b5 (9e4e230b6d3e)
- Not reproducible in Version 5.3.2f1 (e87ab445ead0), Version 5.2.4f1 (98095704e6fe)