Steps to reproduce:

1. Open attached project (link bellow)
2. Open scene "golem totally awesome"
3. Select "golem mecanim" in hierarchy
4. In its Avatar component, click on "Avatar" field to be redirected to the avatar in project tab
5. Click on "golem mecanimAvatar" in project tab and click "Configure Avatar"
6. Avatar configuration scene is opened but bones aren't expanded and two exceptions are thrown in console:
"NullReferenceException: Object reference not set to an instance of an object
UnityEditor.TreeViewUtility.SetChildParentReferences (System.Collections.Generic.List`1 visibleItems, UnityEditor.TreeViewItem root)......"
"InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Peek () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections/Stack.cs:321)....."

Expected behavior: Bones are expanded and no errors are printed in console

Note: this bug is very "picky" when trying to reproduce it:
1. Deleting any object from the scene makes this bug not reproducible
2. Selecting "golem mecanimAvatar" directly from project tab (without being redirected to it like in step 4) makes this bug not reproducible
3. This bug is only reproducible one. To reproduce it again, one needs to reopen project

Regression introduced in 5.3.0b2