ERROR : "Object reference not set to an instance of an object." แนะนำด้วยครับ

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

Private Sub GridViewGTData()
        If IsNothing(Session("GeneralTestTable")) Then
            dt = New DataTable()
            dt.Columns.Add(New DataColumn("Clean", GetType(String)))
            dt.Columns.Add(New DataColumn("Infect", GetType(String)))
            dt.Columns.Add(New DataColumn("Printing", GetType(String)))
            dt.Columns.Add(New DataColumn("GTAVG", GetType(String)))
            dt.Columns.Add(New DataColumn("GTNote", GetType(String)))
            Session("GeneralTestTable") = dt
        End If
        dt = DirectCast(Session("GeneralTestTable"), DataTable)
        gvRMRT_GT.DataSource = dt
        gvRMRT_GT.DataBind()
End Sub

Private Sub AddToTableGT()
        dr = dt.NewRow
        dr("Clean") = rdbClean.SelectedItem.ToString()
        dr("Infect") = rdbInfect.SelectedItem.ToString()
        dr("Printing") = rdbPrinting.SelectedItem.ToString()
        If rdbClean.SelectedItem.Value <> "0" And rdbInfect.SelectedItem.Value <> "0" And rdbPrinting.SelectedItem.Value <> "0" Then
            dr("GTAVG") = "ผ่าน"
        Else
            dr("GTAVG") = "ไม่ผ่าน"
        End If
        dr("GTNote") = txtRMRT_GT_Note.Text.Trim
        dt.Rows.Add(dr)
        Session("GeneralTestTable") = dt
        GridViewGTData()
End Sub

Protected Sub btnRMRT_GT_Add_Click(sender As Object, e As EventArgs) Handles btnRMRT_GT_Add.Click
        AddToTableGT()
End Sub

Private Sub AddToDB()
        ConnectDB()
        GridViewGTData()
        strSQL = "INSERT INTO GeneralTest (GT_CLEAN,GT_INFECT,GT_PRINTING,GT_NOTE) VALUES ('" & dt.Rows(i)        ("Clean") & "','" & dt.Rows(i)("Infect") & "','" & dt.Rows(i)("Printing") & "','" & dt.Rows(i)("GTNote") & "')"
objCmd = New SqlCommand(strSQL, objConn)
            With objCmd
            .Connection = objConn
            .CommandText = strSQL
            .CommandType = CommandType.Text
            .ExecuteNonQuery()
            End With
            Next
            objConn.Close()
            MsgBox("บันทึกข้อมูลสำเร็จ")
End Sub