Steps to Reproduce:
1. Create new Blend tree in animator.
2. Select Blend type - 2D Simple Directional
3. Add new field in Motion.
4. Add one more new field in Motion.
5. Ctr+z and observe the console for the errors

Note:
1. If Blend Type is selected as 1D it is working fine
2. Please see attached screenshot.

Actual Result: Blend tree user interface disappear

Expected Result: Blend tree user interface should not disappear

Reproduced on: 2017.3.0b9,2017.3.0b1, 2017.2.0f3, 2017.1.0f1

Work Around: Instead of using Ctr+z, remove the field by clicking on "-" sign.

License Type - Free
Platform - Windows

StackTrace 1: IndexOutOfRangeException: Array index is out of range.
UnityEditor.BlendTreeInspector.BlendGraph2D (Rect area) (at C:/buildslave/unity/build/Editor/Mono/Inspector/BlendTreeInspector.cs:1089)
UnityEditor.BlendTreeInspector.OnInspectorGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/BlendTreeInspector.cs:354)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1245)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

StackTrace 2: GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)