Steps to reproduce:
1. Create a new project, or alternatively download SkyboxBug.zip project
2. Open the Lighting Settings window
3. Navigate to Environment > Skybox Material
4. Either remove or change the default skybox material (Default_Skybox).

Expected behavior:
Changes to the Skybox Material should be reflected both in Scene and Game views.

Actual behavior:
Changes to the default skybox material are not reflected in either Scene and/or Game views. Changing Environment Lighting source does not have any effect either.

Regression introduced in 2018.1.0b6 (2c4679632cfb)

Note:
- Confirmed working in this version - 2018.1.0b5 (a48da4f646ae)
- Not reproducible in 2018.2.0a1, 2018.1.0b5;
- The issue is also true for all settings under the Environment rollout of the Lighting window.