Объект ThumbnailCard имеет конструктор:
/// <summary>
/// Initializes a new instance of the ThumbnailCard class.
/// </summary>
/// <param name="title">Title of the card</param>
/// <param name="subtitle">Subtitle of the card</param>
/// <param name="text">Text for the card</param>
/// <param name="images">Array of images for the card</param>
/// <param name="buttons">Set of actions applicable to the current
/// card</param>
/// <param name="tap">This action will be activated when user taps on
/// the card itself</param>
public ThumbnailCard(string title = default(string), string subtitle = default(string), string text = default(string), IList<CardImage> images = default(IList<CardImage>), IList<CardAction> buttons = default(IList<CardAction>), CardAction tap = default(CardAction))
{
Title = title;
Subtitle = subtitle;
Text = text;
Images = images;
Buttons = buttons;
Tap = tap;
CustomInit();
}
Вы пробовали с images = new List<CardImage>()
? то же самое для кнопок