Testing Multiplayer Shootout game in editor will show you problem. network error that is produced and then destroys session for client also removes error widget from viewport making it impossible to test Shootout game in editor.
This is a known issue in 4.8 using Multiplayer Shootout sample project, reported here (UE-17889):
There were some changes with how widgets work that have made Client go black with no cursor in this project, and thus no way to join game. There’s a workaround in that post using shortcuts that at least lets you test gameplay. Otherwise, if you want to test in Editor, you can hook your Join functionality up directly after Find Sessions’ first result, and do Find Sessions using a key press event.