On MacOS X when deleting an animation from the blend tree motions following error was received at the console

"InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop"

Steps to repo :

1. Create a animator controller
2. Open the same in Animator Window
3. Create a new state from blend tree
4. Open the blend tree
5. Add motion and blend tree to the same
6. Delete a blend tree and see the error at console

Actual Result:
Error at the console

Expected Result:
Operation should finish smoothly without any errors

Frequency:
Always

Version Tested : 2017.3 Beta 7, 2017.3 Beta 6, 2017.3 Beta 5

Version Where Error Does Not Occur : 2017.1 , 2017.2

Stack trace :

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEngine.GUILayout.EndScrollView (Boolean handleScrollWheel) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:455)
UnityEditor.EditorGUILayout.EndScrollView () (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:8075)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:394)
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)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
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:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:295)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:288)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:255)