Steps to reproduce:

1. Open Unity;
2. Create new project and scene;
3. Add Terrain GameObject (GameObject -> 3DObject ->Terrain);
4. Go to Assets folder (Project tab -> Assets);
5. Delete terrain asset file (by default it's named as "New Terrain") by right-clicking on it and pressing "Delete" button;
6. Double click on Terrain gameobject in the Hierarchy view.

Actual result:

Error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.TerrainInspector.OnGetFrameBounds () (at /Users/builduser/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1530)

is shown in the console. See attached gif video and actual.png screenshot for full error.

Expected result:
- NullReference error should not be shown;
- In this case, it would be reasonable to show a warning message that Terrain has no asset file attached to it every time user clicks on it.

Regression from Unity 5.0.

Notes:
- Reproduces in 5.3.0b5, 5.2.2p2;
- Doesn't reproduce in 4.6.8p3.