To reproduce:
1. Create new project
2. Open Look Dev windows
3. Open Look Dev Environment Library selection window
4. Selection windows is black and null reference exceptions are thrown in console

Reproducible: 5.6.0a1
Regression from 5.5.0b10

Exception:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ObjectListArea+LocalGroup.RefreshBuiltinResourceList (UnityEditor.SearchFilter searchFilter) (at C:/buildslave/unity/build/Editor/Mono/ObjectListLocalGroup.cs:979)
UnityEditor.ObjectListArea+LocalGroup.UpdateFilter (HierarchyType hierarchyType, UnityEditor.SearchFilter searchFilter, Boolean foldersFirst) (at C:/buildslave/unity/build/Editor/Mono/ObjectListLocalGroup.cs:946)
UnityEditor.ObjectListArea.Init (Rect rect, HierarchyType hierarchyType, UnityEditor.SearchFilter searchFilter, Boolean checkThumbnails) (at C:/buildslave/unity/build/Editor/Mono/ObjectListArea.cs:245)
UnityEditor.ObjectSelector.FilterSettingsChanged () (at C:/buildslave/unity/build/Editor/Mono/ObjectSelector.cs:233)
UnityEditor.ObjectSelector.SearchArea () (at C:/buildslave/unity/build/Editor/Mono/ObjectSelector.cs:422)
UnityEditor.ObjectSelector.OnObjectGridGUI () (at C:/buildslave/unity/build/Editor/Mono/ObjectSelector.cs:748)
UnityEditor.ObjectSelector.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/ObjectSelector.cs:710)
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 C:/buildslave/unity/build/Editor/Mono/HostView.cs:249)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:242)
UnityEditor.HostView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:76)