将值设置为属性

Question

嗨，我有一个INotifyPropertyChanged属性，而我想为该属性设置一个值，我得到了null引用异常，为什么会这样？我该如何解决这个问题？我需要在静态类中使用此属性，所以我已经创建了像这样的HolidayTool类

namespace Microsoft.Windows.Controls
{
    public class Calendar : Control
    {
public Calendar()
        {
            HolidayTool.HolidayTool.holy.HolidyContent = "test";

        }
}
}
namespace HolidayTool
{
    public class HolidayTool : INotifyPropertyChanged
    {
        internal static HolidayTool holy;
        public event PropertyChangedEventHandler PropertyChanged;

        public HolidayTool()
        {
            holy = this;
        }
        protected void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private string _HolidyContent;

        public string HolidyContent
        {
            get => _HolidyContent;
            set
            {
                if (value != _HolidyContent)
                {
                    _HolidyContent = value;
                    OnPropertyChanged("HolidyContent");
                }
            }
        }
    }
}