VS2005 中选择gridview1的某行，gridview2显示其他信息的有关问题

    Protected Sub GridView1_SelectedIndexChanging(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSelectEventArgs) Handles GridView1.SelectedIndexChanging
        Dim comm As Data.SqlClient.SqlCommand
        Dim da As Data.SqlClient.SqlDataAdapter
        Dim dt, su As Data.DataTable
        Dim xsdd As String ‘第一个gridview的行号变量
        xsdd = Me.GridView1.SelectedRow.Cells(3).Text()
        '这里报错，“[color=#FF0000]未将对象引用设置到对象的实例[/color]”
        comm = New Data.SqlClient.SqlCommand("SELECT sp_xsdh,sp_mx,sp_sl,sp_dj,sp_je,sp_bz,hkrq,fpje,kprq,kpdw,fph FROM spmx WHERE sp_xsdh = '" & xsdd & "'", conn)
        da = New Data.SqlClient.SqlDataAdapter
        su = CType(GridView2.DataSource, Data.DataTable)
        'su.AcceptChanges()
        comm.CommandType = Data.CommandType.Text
        da.SelectCommand = comm
        dt = New Data.DataTable
        da.Fill(dt)
        GridView2.DataSource = dt
        'Dim r As Data.DataRow = su.NewRow
        'r.Item(0) = "合计："
        'r.Item("sp_je") = su.Compute("sum(sp_je)", "")
        'su.Rows.Add(r)
        GridView2.DataBind()
    End Sub