On opening, prefab editor for a newly created terrain base or variant prefab throws a null reference expectation

Steps to Repro:
1. Open an attached or create a new project
2. Hierarchy > right click
3. 3D Object > terrain
4. drag and drop terrain in the project window to create a prefab
5. Select the prefab and click "Open Prefab" button

Actual Result:
1. Observe console for error
2. The error occurs first time only

Occurring with:
Version 2018.3.0a2 (e9113db2e9bf)
Branch: prefabs/improved

Environment:
Only on Windows

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.TerrainInspector.Raycast (UnityEngine.Vector2& uv, UnityEngine.Vector3& pos) (at C:/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1612)
UnityEditor.TerrainInspector.OnGetFrameBounds () (at C:/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1641)
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.SceneView.FrameSelected (Boolean lockView, Boolean instant) (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2684)
UnityEditor.SceneViewStageHandling.DelayedFraming () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneViewStageHandling.cs:175)
UnityEditor.EditorApplication.Internal_CallUpdateFunctions () (at C:/buildslave/unity/build/Editor/Mono/EditorApplication.cs:197)