"NullReferenceException: Object reference not set to an instance of an object" error appears in the console when trying to create a second animation clip for the same object, then cancelling.

Steps to Reproduce:
1. Create a new project.
2. Open Animation Window.
3. Select any GameObject in the hierarchy.
4. Create an AnimationClip, Save.
5. In the Animation Window, try to create another animation using the dropdown for the same GameObject.
6. When "Create New Animation" window pops up, click Cancel...
if error did not appear, repeat step6 a second time...

error appears in console:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:296)
UnityEngine.GUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:286)
UnityEngine.GUILayout.BeginHorizontal (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:276)
UnityEditor.AnimEditor.OnAnimEditorGUI (UnityEditor.EditorWindow parent, Rect position) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimEditor.cs:170)
UnityEditor.AnimationWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimationWindow.cs:82)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)

Notes:
- Appears only on Mac OSX.
- Appears always.

Please see attached video to reproduce the bug.(NullReference(AnimationClip).mov)

Regression in...
Version 5.6.0b2 (5d236ef0678a)

NOT repro on...
Version 5.6.0b1 (d37b23d9dfff)