How to check if a pack/component Uri exists in a NuGet package?

Question

How can I check if a Uri (file) exists in a referenced project?

Per this answer I've tried catching a FileNotFound exception but it isn't throwing any exceptions at all

string x;

try {
    var Uri = new Uri("/FamFamFam.Flags.Wpf;component/Images/some_wrong_file.png",
        UriKind.Relative);
    x = "a";
}

catch(Exception) {
    x = "b";
}

MessageBox.Show(x);

Per an MSDN thread, I've tried checking if the resultant Bitmap is null, but it always shows 'a':

string x;

try {
    var Bitmap = new BitmapImage(new Uri(
        "/FamFamFam.Flags.Wpf;component/Images/some_wrong_file.png", UriKind.Relative));
    Flag_Image.Source = Bitmap ?? throw new Exception();
    x = "a";
}

catch(Exception) {
    Flag_Image.Source = new BitmapImage(new Uri(
        "/FamFamFam.Flags.Wpf;component/Images/ca.png", UriKind.Relative));
    x = "b";
}

MessageBox.Show(x);