fix null reference when focusing textbox in attached to visual tree,
fix null reference when focusing textbox in attached to visual tree,

but before ontemplate applied.
wieslawsoltes/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,MrDaedra/Avalonia,wieslawsoltes/Perspex,wieslawsoltes/Perspex,grokys/Perspex,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,grokys/Perspex,SuperJMN/Avalonia,AvaloniaUI/Avalonia,MrDaedra/Avalonia,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,Perspex/Perspex,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,jazzay/Perspex,wieslawsoltes/Perspex,jkoritzinsky/Perspex,AvaloniaUI/Avalonia,SuperJMN/Avalonia,akrisiun/Perspex,Perspex/Perspex,wieslawsoltes/Perspex