Fix exception in HtmlControl.
Fix exception in HtmlControl.

jkoritzinsky/Perspex,jkoritzinsky/Avalonia,grokys/Perspex,SuperJMN/Avalonia,MrDaedra/Avalonia,jazzay/Perspex,AvaloniaUI/Avalonia,akrisiun/Perspex,AvaloniaUI/Avalonia,susloparovdenis/Perspex,Perspex/Perspex,SuperJMN/Avalonia,wieslawsoltes/Perspex,OronDF343/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,wieslawsoltes/Perspex,susloparovdenis/Perspex,OronDF343/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,grokys/Perspex,punker76/Perspex,AvaloniaUI/Avalonia,Perspex/Perspex,wieslawsoltes/Perspex,wieslawsoltes/Perspex,wieslawsoltes/Perspex,MrDaedra/Avalonia,jkoritzinsky/Avalonia,susloparovdenis/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,susloparovdenis/Avalonia,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia