您Assosiactions列表需要将项目添加到它之前被实例化。

Your Assosiactions List needs to be instantiated before adding items to it.

***做法是从构造方法中做到这一点。

Best practice is to do this from within the constructor.

public class KeyDate
{
...
    public virtual ICollection<Association> Associations { get; set; }
    public KeyDate()
    {
       Associations = new List<Association>()
    }
}