Steps to repro:

1. Open attached project and 't1' scene;
2. Open Lighting window (Window>Lighting>Settings);
3. Open 'Global Maps' tab in the Lighting window;
4. In the Project folder got to Assets>t1 folder;
5. Delete 'Lightmap-0_comp_light.exr' lightmap from the Project folder.

Expected result:
The lightmap texture is removed without errors in the Console.

Actual result:
- Lighting window UI is corrupted;
- NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EditorGUI.DrawPreviewTextureInternal (Rect position, UnityEngine.Texture image, UnityEngine.Material mat, ScaleMode scaleMode, Single imageAspect, Single mipLevel) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:5314)
UnityEditor.EditorGUI.DrawPreviewTexture (Rect position, UnityEngine.Texture image, UnityEngine.Material mat, ScaleMode scaleMode, Single imageAspect) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:6025)
is shown.
See attached video.

Notes:
- Reproducible in 2018.2.0a1, 2018.1.0b5, 2017.3.0p2, 2017.2.1p3;
- Reproducible on Windows 10 and OSX 10.13.2;
- The issue happens only when Lighting window's Global tab is active and is not happening for other Editor preview windows.