WPF: Cannot set the converter - error when starting the app

Try referencing your converter as a StaticResource

<TextBlock Visibility="{Binding Path=UrlAddress, Converter={StaticResource visibilityConverter}}"> 

forgot to mention Binding Markup Extension with ElementName attribute maybe?

<TextBlock Visibility="{Binding ElementName=XXX, Path=UrlAddress, Converter={StaticResource visibilityConverter}}">  `