Steps to reproduce:
1. Open "uNet" project
2. Build project
3. Open "Loader" scene in Unity Editor.
4. Start the standalone executable and click on "Crear una partida" (start new game).
5. Start the project in Unity Editor and click on "Unirse a una partida" (join an existing game).
-- Both games will show the game scene synced.
6. Close the standalone executable.
-- The game in the Unity Editor will display the start menu with the two buttons.
7. Start the standalone executable and click on "Crear una partida" (start new game).
8. In Unity Editor, click on "Unirse a una partida" (join an existing game).

Results: Editor crashes

Reproduced with: 5.4.0b21, 5.3.5p3, 5.2.4f1