Steps to repro:

1. Open attached project;
2. Open 't1LightProbeGroup' scene;
3. Wait until light baking finishes;
4. Open 't1LightProbeGroupBugPlayMode' scene;
5. In the Hierarchy select 'Light Probe Group' gameobject in the Hierarchy;
6. Enter Play mode;
7. Exit Play mode;
8. Enter Play mode again.

Expected result:
No errors are shown in the console.

Actual result:

Error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.LightProbeGroupInspector.OnDisable () (at C:/buildslave/unity/build/Editor/Mono/Inspector/LightProbeGroupInspector.cs:602)
UnityEditor.InspectorWindow:OnEnable()

is shown after entering Play mode. See attached video.

Regression introduced in 2018.1.0a3.

Notes:
- Reproducible in 2018.1.0a5, 2018.1.0a4, 2018.1.0a3;
- Not reproducible in 2018.1.0a2, 2017.3.0b10, 2017.2.0p2;
- Reproducible on Windows 10 and OSX 10.13.1;
- The issue might not happen every time. If it doesn't happen try entering play mode more times or disabling Light Probe Group component on Light Probe Group gameobject and entering play mode. Finally, you can reopen project and repeat abovementioned steps to trigger the bug;
- The bug's underlying reason might be that in the project there are two identic scenes, except that one has enabled Light Probe Group Component, another doesn't. Toggling this component on one of the scenes might lead to this race condition. The bug doesn't repro when having only one scene with Light Probe Group component.