Null check in ItemsControl on Materialized and Dematerialized, to prevent 
Null check in ItemsControl on Materialized and Dematerialized, to prevent 

exceptions being thrown, trying to do is check on null.
wieslawsoltes/Perspex,MrDaedra/Avalonia,wieslawsoltes/Perspex,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,grokys/Perspex,wieslawsoltes/Perspex,Perspex/Perspex,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,jazzay/Perspex,jkoritzinsky/Perspex,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,AvaloniaUI/Avalonia,MrDaedra/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,akrisiun/Perspex,AvaloniaUI/Avalonia,grokys/Perspex,SuperJMN/Avalonia,Perspex/Perspex,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia