Add null check when popping backgrounds from the mode stack.
Add null check when popping backgrounds from the mode stack.

RedNesto/osu,UselessToucan/osu,johnneijzen/osu,NeoAdonis/osu,tacchinotacchi/osu,EVAST9919/osu,DrabWeb/osu,peppy/osu,ppy/osu,2yangk23/osu,default0/osu,naoey/osu,UselessToucan/osu,ZLima12/osu,Nabile-Rahmani/osu,peppy/osu,osu-RP/osu-RP,smoogipoo/osu,DrabWeb/osu,ppy/osu,NeoAdonis/osu,Drezi126/osu,theguii/osu,NeoAdonis/osu,Damnae/osu,EVAST9919/osu,Frontear/osuKyzer,ZLima12/osu,ppy/osu,smoogipoo/osu,smoogipoo/osu,peppy/osu-new,NotKyon/lolisu,DrabWeb/osu,smoogipooo/osu,2yangk23/osu,peppy/osu,nyaamara/osu,johnneijzen/osu,UselessToucan/osu,naoey/osu,naoey/osu