Steps to repro:

1. Create new project;
2. Add Terrain gameobject (GameObject -> 3D Object -> Terrain);
3. Press Ctrl/cmd+Z or Edit -> Undo Create terrain.

Expected result:
Terrain is deleted without errors.

Actual result:

Error:
MissingReferenceException: The object of type 'Terrain' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEditor.TerrainInspector.OnSceneGUICallback (UnityEditor.SceneView sceneView) (at C:/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1803)
UnityEditor.SceneView.CallOnSceneGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2397)
UnityEditor.SceneView.HandleSelectionAndOnSceneGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1715)
UnityEditor.SceneView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1585)
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)

is shown in the console.
Regression introduced in Unity 5.6.0a6.

Notes:
- Reproducible in 2017.1.0b1, 5.6.0p1, 5.6.0f3, 5.6.0b6;
- Not reproducible in 5.6.0a5, 5.5.2p1;
- Reproducible on both Windows 10 and OSX 10.12.3.