Reproduction steps:
1. Open project attached (ServerExitBug.zip)
2. Open scene "MainMenu" (Assets -> Contect -> Scenes)
3. Build project for Windows (Make sure development build is not marked).
4. Run it.
5. Create server (H).
6. Press stop (X).

Expected result: Server will stop and player will return to MainMenu.
Actual result: Game crashes.

Reproduced on 5.6.0a6, 5.6.0b6.
Works fine on 5.5.0f3, 5.5.1p1.
Regression introduced in 5.6