Fix crash when destroying widget that contains xwt-made backends
Fix crash when destroying widget that contains xwt-made backends

akrisiun/xwt,mono/xwt,hamekoz/xwt,hwthomas/xwt,TheBrainTech/xwt,cra0zy/xwt,residuum/xwt,antmicro/xwt,lytico/xwt