对象引用未设置为对象脚手架mvc的实例

Question

我有一个问题，脚手架我的模型。 我得到以下错误“对象引用未设置为对象的实例”

我的模特：

public class Position
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public long PositionID { get; set; }
    public long OfferID { get; set; }
    [Required]
    [Display(Name = "Positie omschrijving")]
    public string Name { get; set; }
    [Required]
    public int PositionNr { get; set; }
    [Required]
    public double WorkingHeight { get; set; }        
    public long PalletTypeID { get; set; }        
    public long MotorTypeID { get; set; }        
    public long Enveriment { get; set; }   
}

我的DBContext：

public class FlowContext : DbContext
{
    public DbSet<Position> Positions { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
        this.Configuration.LazyLoadingEnabled = true;

        //Positie
        modelBuilder.Entity<Position>()
            .HasKey(j => j.PositionID);
    }
}

我在web.config中的连接字符串

<connectionStrings>
    <add name="FlowContext" connectionString="data source=.\SqlExpress; Integrated Security=True;Initial Catalog=Flow;" providerName="System.Data.SqlClient" />