NullReferenceException when pressing Play with the Sprite Editor open, on OSX

See attached gif

1. Open attached project
2. Expand the texture
3. Drag any sliced sprite onto the scene
4. Open the Sprite Editor, and keep it on the same screen as the editor
5. Press Play

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.LayoutFromContainer (Single w, Single h) (at /Users/weswong/Work/trunk/unity/Runtime/IMGUI/Managed/GUILayoutUtility.cs:182)
UnityEngine.Experimental.UIElements.UIElementsUtility.EndContainerGUI () (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/UIElementsUtility.cs:173)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/IMGUIContainer.cs:110)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleEvent (UnityEngine.Event evt, UnityEngine.Experimental.UIElements.VisualElement finalTarget) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/IMGUIContainer.cs:137)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoRepaint (IStylePainter painter) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/IMGUIContainer.cs:36)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentClip, Matrix4x4 clipTransform, Rect currentGlobalClip) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/Panel.cs:390)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentClip, Matrix4x4 clipTransform, Rect currentGlobalClip) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/Panel.cs:398)
UnityEngine.Experimental.UIElements.Panel.Repaint (UnityEngine.Event e) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/Panel.cs:411)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (IVisualElementPanel panel) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/UIElementsUtility.cs:203)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at /Users/weswong/Work/trunk/unity/Runtime/UIElements/Managed/UIElementsUtility.cs:67)
UnityEngine.GUIUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at /Users/weswong/Work/trunk/unity/Runtime/IMGUI/Managed/GUIUtility.cs:186)

Note: on 5.6 versions, you have to enter Play, exit, enter Play to repro this NullReferenceException
Cannot repro on Win10

Possibly related to https://fogbugz.unity3d.com/f/cases/895220/

Occurring on: 5.6.0a5 onwards till 2017.1.0b2 (current latest)
Not occurring: 5.6.0a4 and older