Editor Shows Missing Reference Exception when user try to configure the Humanoid Character.

Expected Result:
The Editor should allow to configure the Avatar and once Done, User would be displayed the Previous loaded Scene.

Actual Result:
Editor shows, Missing Reference Exception and Does not load the previously loaded Scene.

Steps to Reproduce:
1. Create a New Project
2. Import the assets from the link : https://oc.unity3d.com/index.php/s/VUJC87MqHTpYkqi
3. Find the "BaseHuman" under the HumanaoidAnimation folder
4. Under the Inspector click on Rig Tab
5. Select "Humanoid" option, located besides Animation Type.
6. Click on Apply.
7. Click on "configure" Button
8. User will be prompted to save the Scene if the scene is not Saved.
9. User will be displayed with the options to configure the Humanaoid Avatar.
10. Click on Done.
11. Check the Console Window, An Exception will be displayed.

Stack Trace:
MissingReferenceException: The object of type 'AvatarEditor' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEditor.AvatarEditor.SwitchToAssetMode () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:523)
UnityEditor.AvatarSubEditor.ApplyRevertGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:144)
UnityEditor.AvatarMappingEditor.OnInspectorGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarMappingEditor.cs:301)
UnityEditor.AvatarEditor.EditingGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:440)
UnityEditor.AvatarEditor.OnInspectorGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:378)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1297)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Frequency: 5/5

Environment Details:
Mac OS : 10.12.6
Unity Editor Version: 2018.1.0a3
License : Pro

Reproduced on:2018.1.0a3

Regression introduced on:2018.1.0a3

Works fine on: 2018.1.0a2, 2017.2.0f3, 5.6.3f1