NullReferenceException error with Root Motion Node at runtime

Overriding existing root motion settings with a Root Motion Node generates errors in the Console at runtime.

NullReferenceException: (null)
UnityEditor.SerializedObject..ctor (UnityEngine.Object[] objs, UnityEngine.Object context) (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/SerializedPropertyBindings.gen.cs:87)
UnityEditor.Editor.GetSerializedObjectInternal () (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:175)
UnityEditor.Editor.get_serializedObject () (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:168)
UnityEditor.AvatarMaskInspector.InitializeSerializedProperties () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/AvatarMaskInspector.cs:265)
UnityEditor.AvatarMaskInspector.OnEnable () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/AvatarMaskInspector.cs:274)

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

repro:
-open attached project
-open “bug” scene
-select Adam_03Crowd in Project window
-in the Animation tab…
(notice that Position Y is set to “Bake Into Pose”. This is required for repro)
-now set Motion > Root Motion Node, to “Bip01 > RootMotion”, apply.
-Click Editor Play… errors

- if does not reproduce the first time set Root Motion Node to "none"
- then disable Y, click apply
- re-enable Y, click apply
- Reassign RootMotion node, click apply
- Click Editor play

repros in Version 5.6.0b7 (01fb54121943)
Branch: 5.6/staging

NOT repro in...Version 5.5.1p3 (828893732fe0)
Branch: 5.5/patch-release