[Mac] Don't assume XWT controls the entire view hierarchy
[Mac] Don't assume XWT controls the entire view hierarchy

We were getting an NRE when we reached a superview that's not an IViewObject

TheBrainTech/xwt,residuum/xwt,iainx/xwt,cra0zy/xwt,mono/xwt,steffenWi/xwt,mminns/xwt,directhex/xwt,hwthomas/xwt,sevoku/xwt,hamekoz/xwt,antmicro/xwt,lytico/xwt,akrisiun/xwt,mminns/xwt