To reproduce:

1. Open attached project.
2. Open scene: AnimationTestScene
3. Select asset in project view: Assets/units/New Experimental/Animations/Maw_J_Laygo@dwarf_walk
4. Reimport it by unticking and ticking again "Import Animation" field and press apply.
5. Press play.
6. Leave playmode, and press play again.

After second playmode enter, an error appears everytime entering playmode:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.AnimationClipInfoProperties.MaskFromClip (UnityEngine.AvatarMask mask) (at C:/buildslave/unity/build/Editor/Mono/ImportSettings/AnimationClipInfoProperties.cs:107)
UnityEditor.AvatarMaskInspector.set_clipInfo (UnityEditor.AnimationClipInfoProperties value) (at C:/buildslave/unity/build/Editor/Mono/Inspector/AvatarMaskInspector.cs:221)
UnityEditor.ModelImporterClipEditor.SyncClipEditor () (at C:/buildslave/unity/build/Editor/Mono/ImportSettings/ModelImporterClipEditor.cs:214)
UnityEditor.ModelImporterClipEditor.OnEnable () (at C:/buildslave/unity/build/Editor/Mono/ImportSettings/ModelImporterClipEditor.cs:176)

or sometimes gives different error (or both):

NullReferenceException: (null)
UnityEditor.SerializedObject..ctor (UnityEngine.Object[] objs, UnityEngine.Object context) (at C:/buildslave/unity/build/artifacts/generated/common/editor/SerializedPropertyBindings.gen.cs:87)
UnityEditor.Editor.GetSerializedObjectInternal () (at C:/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:175)
UnityEditor.Editor.get_serializedObject () (at C:/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:168)
UnityEditor.AvatarMaskInspector.InitializeSerializedProperties () (at C:/buildslave/unity/build/Editor/Mono/Inspector/AvatarMaskInspector.cs:265)

Reproducible on: 5.6.0b7, 5.6.0b5 (802d0eaaa43c)
Not reproducible on: 5.6.0b1 (d37b23d9dfff)

Regression introduced somewhere between 5.6.0b5 and 5.6.0b1