回发时未触发ASP.NET BasePage类页面加载

我有下面的BasePage类。。。

Public Class BasePage
    Inherits System.Web.UI.Page

    Private litError As Literal
    Protected SO As Session

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        SO = Session.Item("SO")

        If SO Is Nothing Then
            Session.Abandon()
            Response.Redirect("~/timeout.htm")
        End If

        litError = Page.FindControl("litError")
        If litError IsNot Nothing Then
            litError.Visible = False
        End If
    End Sub

    Protected Sub ShowMessage(ByVal Message As String)
        Show(Message, "message")
    End Sub

    Protected Sub ShowError(ByVal Message As String)
        Show(Message, "error message")
    End Sub

    Protected Sub ShowSuccess(ByVal Message As String)
        Show(Message, "success message")
    End Sub

    Private Sub Show(ByVal Message As String, ByVal CssClass As String)
        If litError IsNot Nothing Then
            litError.Text = String.Format("<span class=""{0}"">{1}</span>", CssClass, HttpUtility.HtmlEncode(Message))
            litError.Visible = True
        End If
    End Sub
End Class

此应用程序中的每个页面都继承此类。SO变量表示一个自定义会话类,该类非常简单,只包含几个在整个应用程序中使用的基本设置。问题是,如果发生自然回发(在本例中,它是通过排序/分页进行的gridview回发),则不会触发此基类中的my Page_加载。后来在我的代码中,当我引用SO时,我得到一个空引用异常,因为它尚未从会话中提取。

为什么基页不加载fire?