Fix WorkingBeatmap disposal potentially null-refing (#5423)
Fix WorkingBeatmap disposal potentially null-refing (#5423)

Fix WorkingBeatmap disposal potentially null-refing
UselessToucan/osu,ZLima12/osu,smoogipoo/osu,ppy/osu,smoogipooo/osu,peppy/osu-new,johnneijzen/osu,peppy/osu,EVAST9919/osu,ppy/osu,UselessToucan/osu,NeoAdonis/osu,peppy/osu,peppy/osu,smoogipoo/osu,UselessToucan/osu,smoogipoo/osu,NeoAdonis/osu,2yangk23/osu,2yangk23/osu,johnneijzen/osu,EVAST9919/osu,ppy/osu,NeoAdonis/osu,ZLima12/osu