vb.net - NullReferenceException - フィールドを更新する必要がある

Question

GridView のフィールドを更新しようとしましたが、成功しませんでした。これは、 Object reference not set to an instance of an objectを得たエラーです。この行で壊れ続けますdt.Rows(row.DataItemIndex)("TicketID") = (CType(row.FindControl("TicketID"), TextBox)).ToString初心者なので、これを修正する方法がわかりません。どんな助けでも大歓迎です。

分離コード:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    Dim dt As New DataTable("historyList")

    dt.Columns.Add("TicketID", GetType(Integer))
    dt.Columns.Add("DateCreated", GetType(DateTime))
    dt.Columns.Add("FullName", GetType(String))
    dt.Columns.Add("TicketType", GetType(String))
    dt.Columns.Add("Subject", GetType(String))
    dt.Columns.Add("Message", GetType(String))
    dt.Columns.Add("Status", GetType(String))

    For i = 0 To 6
        Dim tableRow = dt.NewRow()
        tableRow("TicketID") = i
        tableRow("DateCreated") = Now()
        tableRow("FullName") = i.ToString()
        tableRow("TicketType") = i.ToString()
        tableRow("Subject") = i.ToString()
        tableRow("Message") = i.ToString()
        tableRow("Status") = i.ToString()
        dt.Rows.Add(tableRow)
    Next

    Session("dt") = dt

    BindData()

End Sub

Protected Sub GridView1_RowEditing(ByVal sender As Object, ByVal e As GridViewEditEventArgs)
    GridView1.EditIndex = e.NewEditIndex
    BindData()
End Sub
Protected Sub GridView1_RowCancelingEdit(ByVal sender As Object, ByVal e As GridViewCancelEditEventArgs)
    GridView1.EditIndex = 1
    BindData()
End Sub

Protected Sub GridView1_RowUpdating(ByVal sender As Object, ByVal e As GridViewUpdateEventArgs)
    Dim dt = CType(Session("dt"), DataTable)

    'if your current DataSource be in Session
    Dim row As GridViewRow = GridView1.Rows(e.RowIndex)

    dt.Rows(row.DataItemIndex)("TicketID") = (CType(row.FindControl("TicketID"), TextBox)).ToString
    dt.Rows(row.DataItemIndex)("DateCreated") = (CType(row.FindControl("DateCreated"), TextBox)).ToString
    dt.Rows(row.DataItemIndex)("FullName") = (CType(row.FindControl("FullName"), TextBox)).ToString
    dt.Rows(row.DataItemIndex)("TicketType") = (CType(row.FindControl("TicketType"), TextBox)).ToString
    dt.Rows(row.DataItemIndex)("Subject") = (CType(row.FindControl("Subject"), TextBox)).ToString
    dt.Rows(row.DataItemIndex)("Message") = (CType(row.FindControl("Message"), TextBox)).ToString
    dt.Rows(row.DataItemIndex)("Status") = (CType(row.FindControl("Status"), TextBox)).ToString

    Session("dt") = dt
    GridView1.EditIndex = 1
    BindData()

End Sub

Private Sub BindData()

    'GridView1.DataSource = Session("dt")
    GridView1.DataBind()

End Sub