It happens only on NetworkLobbyManager component (doesn't happen on NetworkManager)

To repro:
1. Create a project.
2. Create an empty object and add Lobby Manager component.
3. Press on wheel near Lobby Player Prefab or Game Player Prefab.

Actual result: Window is corrupted. Errors in log.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ObjectListArea+LocalGroup.RefreshBuiltinResourceList (UnityEditor.SearchFilter searchFilter) (at /Users/builduser/buildslave/unity/build/Editor/Mono/ObjectListLocalGroup.cs:974)

Expected result: Window opens fine. No errors in log.

Regression since 5.6 (does not repro in 5.5rc2), reproduces in 5.6.0a2 and 5.6.0a3.