Getting an error when entering (double-click) a Blend Tree in the Animator window.

repro:
-create a new project
-create an Animator controller
-select controller
-open Animator
-RMB-click > Create State > From New Blend Tree
-Double-click the Blend Tree node to enter it... error...

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.GraphGUI.GetCenterPosition () (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/GUI/GraphGUI.cs:35)
UnityEditor.Graphs.GraphGUI.CenterGraph () (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/GUI/GraphGUI.cs:912)
UnityEditor.Graphs.AnimatorControllerTool.CenterGraph () (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:675)
UnityEditor.Graphs.AnimatorControllerTool.CenterOnStoredPosition (UnityEngine.Object target) (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:700)
UnityEditor.Graphs.AnimatorControllerTool.AddBreadCrumb (UnityEngine.Object target, Boolean updateViewPosition) (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:622)
UnityEditor.Graphs.AnimationStateMachine.StateNode.NodeUI (UnityEditor.Graphs.GraphGUI host) (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/StateNode.cs:49)
UnityEditor.Graphs.AnimationStateMachine.GraphGUI.NodeGUI (UnityEditor.Graphs.Node n) (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/GraphGUI.cs:124)
UnityEditor.Graphs.AnimationStateMachine.GraphGUI+<OnGraphGUI>c__AnonStorey0.<>m__0 (Int32 ) (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/GraphGUI.cs:350)
UnityEngine.GUILayout+LayoutedWindow.DoWindow (Int32 windowID) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:497)
UnityEngine.GUI.CallWindowDelegate (UnityEngine.WindowFunction func, Int32 id, Int32 instanceID, UnityEngine.GUISkin _skin, Int32 forceRect, Single width, Single height, UnityEngine.GUIStyle style) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUI.cs:1817)
UnityEditor.DockArea:OnGUI()

repro in...
Version 5.6.0f1 (95292252f04a)
Branch: 5.6/staging

not repro in 5.5.2p2