Regression: first build where issue was introduced: Version 5.4.0b7 (b558e46def43)
Steps:
1. Open attached project
2. Go to "Assets -> Dialogue System -> Examples -> Art -> Recon Troop -> Model"
3. Select "mReconTroop.fbx" model
==>
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.AvatarMaskInspector.IsMaskUpToDate () (at C:/buildslave/unity/build/Editor/Mono/Inspector/AvatarMaskInspector.cs:370)

In previous beta (before b7) was another error:
GetLocalizedString is not allowed to be called from a MonoBehaviour constructor, call it in Awake or Start instead. Called from script 'AvatarMaskInspector' on game object ''.
See "Script Serialization" page in the Unity Manual for further details.
UnityEditor.ModelImporterEditor:OnEnable()