Fix potential nullref when exiting multiplayer (#4227)
Fix potential nullref when exiting multiplayer (#4227)

Fixes https://github.com/ppy/osu/issues/3954
UselessToucan/osu,ppy/osu,peppy/osu,2yangk23/osu,smoogipoo/osu,DrabWeb/osu,ppy/osu,EVAST9919/osu,smoogipooo/osu,naoey/osu,ppy/osu,johnneijzen/osu,ZLima12/osu,UselessToucan/osu,naoey/osu,smoogipoo/osu,johnneijzen/osu,EVAST9919/osu,ZLima12/osu,NeoAdonis/osu,DrabWeb/osu,peppy/osu-new,DrabWeb/osu,NeoAdonis/osu,NeoAdonis/osu,smoogipoo/osu,UselessToucan/osu,peppy/osu,2yangk23/osu,peppy/osu,naoey/osu