在构造函数调用会话中，对象引用未设置为对象的实例

Question

public AccountController()
{
    SchoolModels SH = new SchoolModels();
    if (**Session["IndependentSchool"]** != null)
    {
        SH = (SchoolModels)Session["IndependentSchool"];
    }
}

Session["IndependentSchool"]  // error here 

如何解决？