On MacOSx NullReferenceException: SerializedObject of SerializedProperty has been Disposed error was thrown at console, when made changes to humanoid type character, muscle limit changes, and clicking on apply pop up , when prompted for saving the changes.

Steps to repo:

1. Open the GDC2013 Mecaqnim sample project,
2. Select the default character and open configuration Tab
3. Open Muscle Setting and make modifications
4. Instead of clicking the apply, double click a scene from project tab
5 .When pop up prompts , click apply and see the console

Actual Results:
Operation completes with error at console

Expected Result:
No Error should prompt up at console

Frequency:Always

ersion 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:

NullReferenceException: SerializedObject of SerializedProperty has been Disposed.
UnityEditor.AvatarMuscleEditor.ResetValuesFromProperties () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarMuscleEditor.cs:448)
UnityEditor.AvatarMuscleEditor.ResetValues () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarMuscleEditor.cs:653)
UnityEditor.AvatarSubEditor.ApplyAndImport () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:115)
UnityEditor.AvatarSubEditor.OnDestroy () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:75)
UnityEditor.AvatarEditor.DestroyEditor () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:333)
UnityEditor.AvatarEditor.SwitchToAssetMode () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:525)
UnityEditor.AvatarEditor.OnDestroy () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:299)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)