添加数据时实体框架错误：System.NullReferenceException：'对象引用未设置为对象的实例。'

Question

我正在开发一个带有实体框架和Csharp的预会计软件。

我得到“System.NullReferenceException：'对象引用未设置为对象的实例。'”尝试时将错误添加到数据库中。

有关错误的实体;

该实体保留客户和供应商的数据。

public class CompanyInfo
{
    public int CompanyInfoId { get; set; }
    public string Title { get; set; }
    public string AddressCity { get; set; }
    public string AddressDistrict { get; set; }
    public string AddressNeighborhood { get; set; }
    public string AddressStreet { get; set; }
    public string AddressNo { get; set; }
    public string AddressFloor { get; set; }
    public string TaxNo { get; set; }
    public string TaxAdministration { get; set; }
    public string AuthorizedPerson { get; set; }
    public bool AuthorizedPersonGender { get; set; }
    public string Tel { get; set; }
    public string Fax { get; set; }
    public string Mail { get; set; }
    public string TradeRegistryNo { get; set; }
    public DateTime CreateDate { get; set; }
    public bool IsActive { get; set; }
    public string Iban { get; set; }
    public bool IsSupplier { get; set; }
    public bool IsCustomer  { get; set; }

    //Navigation Property
    public virtual UserCompany UserCompany{ get; set; }
    public virtual ICollection<Invoice> Invoices { get; set; }
    public virtual ICollection<PeriodicService> PeriodicServices{ get; set; }
    public virtual ICollection<FinancialTransaction> FinancialTransactions { get; set; }

    public CompanyInfo()
    {
        Invoices = new HashSet<Invoice>();
        PeriodicServices = new HashSet<PeriodicService>();
        FinancialTransactions = new HashSet<FinancialTransaction>();
    }
}

此实体保留用户公司的数据。

public class UserCompany
{
    public int UserCompanyId { get; set; }
    public string Title { get; set; }
    public string AdressCity { get; set; }
    public string AdressNeighborhood { get; set; }
    public string AdressDistrict { get; set; }
    public string AdressStreet { get; set; }
    public string AdressNo { get; set; }
    public string AdressFloor { get; set; }
    public string TaxNo { get; set; }
    public string TaxAdministration { get; set; }
    public string LogoPath { get; set; }
    public string Tel { get; set; }
    public string Fax { get; set; }
    public string Mail { get; set; }
    public string TradeRegistryNo { get; set; }
    public DateTime CreateDate { get; set; }
    public bool IsActive { get; set; }
    public string Iban { get; set; }

    // Navigation Property
    public virtual ICollection<ProgramUser> ProgramUsers { get; set; }
    public virtual ICollection<CompanyInfo> CompanyInfos { get; set; }
    public virtual ICollection<PeriodicService> PeriodicServices { get; set; }
    public virtual ICollection<FinancialTransaction> FinancialTransactions { get; set; }
    public virtual ICollection<Invoice> Invoices { get; set; }
    public virtual ICollection<Service> Services  { get; set; }
    public virtual ICollection<Stock> Products { get; set; }

    public UserCompany()
    {
        ProgramUsers = new HashSet<ProgramUser>();
        CompanyInfos = new HashSet<CompanyInfo>();
        PeriodicServices = new HashSet<PeriodicService>();
        FinancialTransactions = new HashSet<FinancialTransaction>();
        Invoices = new HashSet<Invoice>();
        Services = new HashSet<Service>();
        Products = new HashSet<Stock>();
    }
}

因此，UserCompany和CompanyInfo之间存在一对多的关系。

此代码用于添加公司并与用户公司建立关系......

public void AddCompany()
    {
        if (CheckRequiredAddCompanyFields())
        {
            CompanyInfo newCompany = new CompanyInfo();
            newCompany.Title = addCompanyForm.mtxtDetailTitle.Text;
            newCompany.AddressCity = addCompanyForm.mtxtCity.Text;
            newCompany.AddressDistrict = addCompanyForm.mtxtDistrict.Text;
            newCompany.AddressNeighborhood = addCompanyForm.mtxtNeighborhood.Text;
            newCompany.AddressStreet = addCompanyForm.mtxtStreet.Text;
            newCompany.AddressNo = addCompanyForm.mtxtAdressNo.Text;
            newCompany.AddressFloor = addCompanyForm.mtxtFloor.Text;
            newCompany.TaxNo = addCompanyForm.mtxtTaxNo.Text;
            newCompany.TaxAdministration = addCompanyForm.mtxtTaxAdministration.Text;
            newCompany.AuthorizedPerson = addCompanyForm.mtxtAuthorizedPerson.Text;
            newCompany.AuthorizedPersonGender = addCompanyForm.mrdbtnMale.Checked ? true : false;
            newCompany.Tel = addCompanyForm.mtxtDetailTel.Text;
            newCompany.Fax = addCompanyForm.mtxtDetailFax.Text;
            newCompany.Mail = addCompanyForm.mtxtMail.Text;
            newCompany.TradeRegistryNo = addCompanyForm.mtxtTradeRegistryNo.Text;
            newCompany.CreateDate = DateTime.Today;
            newCompany.IsActive = true;
            newCompany.Iban = addCompanyForm.mtxtIban.Text;
            newCompany.IsCustomer = addCompanyForm.mcboxDetailCustomer.Checked == true ? true : false;
            newCompany.IsSupplier = addCompanyForm.mcboxDetailSupplier.Checked == true ? true: false;

            //The error was thrown from the following line
            newCompany.UserCompany.UserCompanyId = mainForm.userCompanyId;
            bCompanyInfo.Add(newCompany); 

            FillCompaniesToDataGrid(mainForm.userCompanyId);
        }

    }