To reproduce:
- Open attached project and launch "test" scene
- Select Main Camera and go to LODGroup component in Inspector
- Click on "Add" to add a renderer and observe that a new window open
- Close the Object Picker window and observe the following error in Console:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.LODGroupEditor <GetRenderers>c__AnonStorey4E.<>m__A7 (UnityEngine.GameObject go) (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/Inspector/LODGroupEditor.cs:559)
System.Linq.Enumerable <CreateWhereIterator>c__Iterator1D`1[UnityEngine.GameObject].MoveNext ()
System.Linq.Enumerable.Count[GameObject] (IEnumerable`1 source)