Hmm, if i add that reference to your project, it fails with
E: Unknown type “{clr-namespace:MaterialDesignThemes.Wpf;assembly=MaterialDesignThemes.Wpf}PackIcon” [C:\Users\mh\Downloads\WPFApplication2\WPFApplication2\Window1.xaml (9)]
Hmm, if i add that reference to your project, it fails with
E: Unknown type “{clr-namespace:MaterialDesignThemes.Wpf;assembly=MaterialDesignThemes.Wpf}PackIcon” [C:\Users\mh\Downloads\WPFApplication2\WPFApplication2\Window1.xaml (9)]