How to reproduce:
1. Download and open attached project "20.06.2018 - Nested Prefab Test"
2. Observe Hierarchy window

Expected result: Scene objects are displayed in Hierarchy window
Actual result: Scene objects are not displayed in Hierarchy window

Reproduced with: 2018.2.0x-ImprovedPrefabs

Notes: also Hierarchy window scroll bar cannot be held down and does not respond to mouse wheel. Plugin used - EModules.

Error in console:
NullReferenceException: Object reference not set to an instance of an object
EModules.EModulesInternal.Hierarchy.InitializeReflection ()
EModules.EModulesInternal.Hierarchy.Constructor ()
EModules.EModulesInternal.Hierarchy.Loopinit ()
EModules.EModulesInternal.Hierarchy..cctor ()
Rethrow as TypeInitializationException: An exception was thrown by the type initializer for EModules.EModulesInternal.Hierarchy
System.Runtime.CompilerServices.RuntimeHelpers.RunClassConstructor (RuntimeTypeHandle type) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Runtime.CompilerServices/RuntimeHelpers.cs:101)
UnityEditor.EditorAssemblies.ProcessEditorInitializeOnLoad (System.Type type) (at C:/buildslave/unity/build/Editor/Mono/EditorAssemblies.cs:93)