Fix workingbeatmap's disposal potentially null-refing
Fix workingbeatmap's disposal potentially null-refing

EVAST9919/osu,ppy/osu,2yangk23/osu,ppy/osu,ZLima12/osu,EVAST9919/osu,smoogipoo/osu,UselessToucan/osu,2yangk23/osu,smoogipoo/osu,peppy/osu-new,ppy/osu,johnneijzen/osu,peppy/osu,peppy/osu,johnneijzen/osu,UselessToucan/osu,peppy/osu,smoogipoo/osu,smoogipooo/osu,ZLima12/osu,NeoAdonis/osu,NeoAdonis/osu,UselessToucan/osu,NeoAdonis/osu