"Asp.net MVC EF 下拉列表 - 对象引用未设置为对象的实例"

Question

我正在尝试创建一个简单的下拉列表。 我可能以任何可能的方式做到了这一点。 我仍然不明白为什么它会失败。 现在我有以下错误：我的剃须刀页面上的“对象引用未设置为对象的实例”。

这是我的模型：

    public class Contacts
    {
        [Key]
        public int Id { get; set; }
        [Display(Name = "Company")]
        public string? Company { get; set; }
        [Required]
        public string Name { get; set; }
        public string? Surname { get; set; }
        [NotMapped]
        public Companies Companies { get; set; }
        [NotMapped]
        public IEnumerable<SelectListItem> CompaniesList { get; set; }
        public Contacts()
        {
            this.Companies = new Companies();
        }
    }

    public class Companies
    {
        [Key]
        public int Id { get; set; }
        [Required]
        [MaxLength(100)]
        [MinLength(3)]
        public string Name { get; set; }
        [MaxLength(100)]
        public string? Description { get; set; }
        [Display(Name = "Tax Identification Number")]
        public string? TaxIdNumber { get; set; }
        public string? Country { get; set; }
        [Display(Name = "Address")]
        public string? StreetAddress { get; set; }
        [Display(Name = "Active")]
        public bool IsActive { get; set; }
        [Display(Name = "Select")]
        public bool CheckBox { get; set; }
    }