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

Imports System.Data.SqlClient
Imports System.Globalization

Public Class Test_Manage_Package
    Inherits System.Web.UI.Page

    Dim objConn As SqlConnection
    Dim objCmd As SqlCommand
    Dim strConnString, strSQL As String
    Dim dtAdapter As SqlDataAdapter
    Dim dt, dtddl As DataTable
    Dim iMAX, Count As Integer

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

        lblPK_AddReceiveDate.Text = Format(Now.Date.ToString("dd/MM/yyyy", New CultureInfo("en-US")))

        If Not IsPostBack Then
            DropdownListRawMaterialType()
            LoadSizeData()
        End If

    End Sub

    Public Sub ConnectDB()

        strConnString = ConfigurationManager.ConnectionStrings("con").ConnectionString
        objConn = New SqlConnection(strConnString)

    End Sub

    Function DropdownListRawMaterialType() As DataTable

        ConnectDB()

        strSQL = "Select RMT_RKEY,RMT_NAME From RawMaterialType WHERE RMT_NAME <> 'แป้ง' AND RMT_NAME <> 'สารเคมี' ORDER BY RMT_NAME ASC"
        dtAdapter = New SqlDataAdapter(strSQL, objConn)
        dt = New DataTable

        objConn.Open()
        dtAdapter.Fill(dt)
        objConn.Close()

        ddlPK_LoadRMT.DataSource = dt
        ddlPK_LoadRMT.DataTextField = "RMT_NAME"
        ddlPK_LoadRMT.DataValueField = "RMT_RKEY"
        ddlPK_LoadRMT.DataBind()
        ddlPK_LoadRMT.Items.Insert(0, New ListItem("กรุณาเลือกประเภทวัตถุดิบ", "0"))
        ddlPK_LoadRM.Items.Insert(0, New ListItem("กรุณาเลือกวัตถุดิบ", "0"))

        ddlPK_LoadRMT.SelectedIndex = ddlPK_LoadRMT.Items.IndexOf(ddlPK_LoadRMT.Items.FindByValue("0"))
        ddlPK_LoadRM.SelectedIndex = ddlPK_LoadRM.Items.IndexOf(ddlPK_LoadRM.Items.FindByValue("0"))
        ddlPK_LoadRM.Enabled = False

        Return dt
    End Function

    Private Sub ddlPK_LoadRMT_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ddlPK_LoadRMT.SelectedIndexChanged

        Try
            ConnectDB()
            'ddlPK_LoadRMT.AppendDataBoundItems = True

            strSQL = "SELECT RM_ID,RM_NAME FROM RawMaterial WHERE RMT_RKEY=@RMT_RKEY ORDER BY RM_NAME ASC"
            objCmd = New SqlCommand(strSQL, objConn)
            objCmd.Parameters.AddWithValue("@RMT_RKEY", ddlPK_LoadRMT.SelectedItem.Value)

            objConn.Open()
            ddlPK_LoadRM.DataSource = objCmd.ExecuteReader()
            ddlPK_LoadRM.DataTextField = "RM_NAME"
            ddlPK_LoadRM.DataValueField = "RM_ID"
            ddlPK_LoadRM.DataBind()
            ddlPK_LoadRM.Items.Insert(0, New ListItem("กรุณาเลือกวัตถุดิบ", "0"))

            ddlPK_LoadRM.SelectedIndex = ddlPK_LoadRM.Items.IndexOf(ddlPK_LoadRM.Items.FindByValue("0"))

            If ddlPK_LoadRM.Items.Count > 1 Then
                ddlPK_LoadRM.Enabled = True
            Else
                ddlPK_LoadRM.Enabled = False
            End If
        Catch ex As Exception
            Throw ex
        Finally
            objConn.Close()
        End Try

    End Sub

    Private Sub LoadSizeData()

        If IsNothing(Session("SizeTable")) Then
            dt = New DataTable()
            dt.Columns.Add(New DataColumn("TopWidth", GetType(String)))
            dt.Columns.Add(New DataColumn("MidWidth", GetType(String)))
            dt.Columns.Add(New DataColumn("BottomWidth", GetType(String)))
            dt.Columns.Add(New DataColumn("WidthAVG", GetType(String)))
            dt.Columns.Add(New DataColumn("LeftLength", GetType(String)))
            dt.Columns.Add(New DataColumn("RigthLength", GetType(String)))
            dt.Columns.Add(New DataColumn("LengthAVG", GetType(String)))
            dt.Columns.Add(New DataColumn("SizeNote", GetType(String)))
            Session("SizeTable") = dt
        End If

        dt = DirectCast(Session("SizeTable"), DataTable)
        gvS_LoadSize.DataSource = dt
        gvS_LoadSize.DataBind()

    End Sub

    Private Sub AddToTable()

        If txtPK_WidthTop.Text.Trim <> String.Empty And txtPK_WidthMid.Text.Trim <> String.Empty And txtPK_WidthBottom.Text.Trim <> String.Empty And txtPK_LengthLeft.Text.Trim <> String.Empty And txtPK_LengthRigth.Text.Trim <> String.Empty Then

            Dim dr As DataRow = dt.NewRow

            dr("TopWidth") = txtPK_WidthTop.Text.Trim
            dr("MidWidth") = txtPK_WidthMid.Text.Trim
            dr("BottomWidth") = txtPK_WidthBottom.Text.Trim
            dr("WidthAVG") = ((CType(txtPK_WidthTop.Text.Trim, Decimal) + CType(txtPK_WidthMid.Text.Trim, Decimal) + CType(txtPK_WidthBottom.Text.Trim, Decimal)) / 3).ToString("n2")
            dr("LeftLength") = txtPK_LengthLeft.Text.Trim
            dr("RigthLength") = txtPK_LengthRigth.Text.Trim
            dr("LengthAVG") = ((CType(txtPK_LengthLeft.Text.Trim, Decimal) + CType(txtPK_LengthRigth.Text.Trim, Decimal)) / 2).ToString("n2")
            dr("SizeNote") = txtPK_SizeNote.Text.Trim

            dt.Rows.Add(dr)
            Session("SizeTable") = dt
            LoadSizeData()
            Reset()
            txtPK_WidthTop.Focus()

        ElseIf txtPK_WidthTop.Text.Trim = String.Empty And txtPK_WidthMid.Text.Trim <> String.Empty And txtPK_WidthBottom.Text.Trim <> String.Empty And txtPK_LengthLeft.Text.Trim <> String.Empty And txtPK_LengthRigth.Text.Trim <> String.Empty Then
            MsgBox("กรุณากรอกความกว้างส่วนบน")
            txtPK_WidthTop.Focus()
        ElseIf txtPK_WidthTop.Text.Trim <> String.Empty And txtPK_WidthMid.Text.Trim = String.Empty And txtPK_WidthBottom.Text.Trim <> String.Empty And txtPK_LengthLeft.Text.Trim <> String.Empty And txtPK_LengthRigth.Text.Trim <> String.Empty Then
            MsgBox("กรุณากรอกความกว้างส่วนกลาง")
            txtPK_WidthMid.Focus()
        ElseIf txtPK_WidthTop.Text.Trim <> String.Empty And txtPK_WidthMid.Text.Trim <> String.Empty And txtPK_WidthBottom.Text.Trim = String.Empty And txtPK_LengthLeft.Text.Trim <> String.Empty And txtPK_LengthRigth.Text.Trim <> String.Empty Then
            MsgBox("กรุณากรอกความกว้างส่วนล่าง")
            txtPK_WidthBottom.Focus()
        ElseIf txtPK_WidthTop.Text.Trim <> String.Empty And txtPK_WidthMid.Text.Trim <> String.Empty And txtPK_WidthBottom.Text.Trim <> String.Empty And txtPK_LengthLeft.Text.Trim = String.Empty And txtPK_LengthRigth.Text.Trim <> String.Empty Then
            MsgBox("กรุณากรอกความยาวด้านซ้าย")
            txtPK_LengthLeft.Focus()
        ElseIf txtPK_WidthTop.Text.Trim <> String.Empty And txtPK_WidthMid.Text.Trim <> String.Empty And txtPK_WidthBottom.Text.Trim <> String.Empty And txtPK_LengthLeft.Text.Trim <> String.Empty And txtPK_LengthRigth.Text.Trim = String.Empty Then
            MsgBox("กรุณากรอกความยาวด้านขวา")
            txtPK_LengthRigth.Focus()
        Else
            MsgBox("กรุณากรอกข้อมูลให้ครบถ้วน")
            txtPK_WidthTop.Focus()
        End If

    End Sub

    Private Sub Reset()

        txtPK_WidthTop.Text = String.Empty
        txtPK_WidthMid.Text = String.Empty
        txtPK_WidthBottom.Text = String.Empty
        txtPK_LengthLeft.Text = String.Empty
        txtPK_LengthRigth.Text = String.Empty
        txtPK_SizeNote.Text = String.Empty

    End Sub

    Protected Sub btnPK_Add_Click(sender As Object, e As EventArgs) Handles btnPK_Add.Click

        AddToTable()

    End Sub

    Private Sub AddTODB()

        LoadSizeData()

        If dt.Rows.Count > 0 Then

            Try
                ConnectDB()
                objConn.Open()

                strSQL = "SELECT ISNULL(MAX(RIGHT(PK_ID,2)),0) As PK_ID FROM Package"
                objCmd = New SqlCommand(strSQL, objConn)

                Count = objCmd.ExecuteScalar()

                iMAX = IIf(Count <= 0, 1, Count + 1)
                If iMAX < 100 Then
                    strSQL = "INSERT INTO Package (PK_ID, PK_RECEIVEDATE) VALUES ('" & "PK" & CStr(Format(Now.Date.ToString("yyyyMMdd", New CultureInfo("en-US")))) & iMAX.ToString("00") & "',@PK_RECEIVEDATE)"
                    objCmd = New SqlCommand(strSQL, objConn)

                    objCmd.Parameters.Add("@PK_RECEIVEDATE", SqlDbType.Date).Value = Format(Now.Date.ToString("dd-MM-yyyy", New CultureInfo("en-US")))
                    objCmd.ExecuteNonQuery()

                    iMAX = IIf(Count <= 0, 1, Count + 1)
                    If iMAX < 100 Then

                        dt = DirectCast(Session("SizeTable"), DataTable)
                        For i = 0 To dt.Rows.Count - 1

                            strSQL = "INSERT INTO Size (PK_ID,S_WIDTHTOP,S_WIDTHMID,S_WIDTHBOTTOM,S_LENGTHLEFT,S_LENGTHRIGTH,S_NOTE) VALUES ('" & "PK" & CStr(Format(Now.Date.ToString("yyyyMMdd", New CultureInfo("en-US")))) & iMAX.ToString("00") & "','" & dt.Rows(i)("TopWidth") & "','" & dt.Rows(i)("MidWidth") & "','" & dt.Rows(i)("BottomWidth") & "','" & dt.Rows(i)("LeftLength") & "','" & dt.Rows(i)("RigthLength") & "','" & dt.Rows(i)("SizeNote") & "')"
                            objCmd = New SqlCommand(strSQL, objConn)


                            With objCmd
                                .Connection = objConn
                                .CommandText = strSQL
                                .CommandType = CommandType.Text
                                .ExecuteNonQuery()
                            End With
                        Next

                        objConn.Close()
                        objConn = Nothing
                        MsgBox("บันทึกข้อมูลสำเร็จ")
                        dt.Clear()
                        Response.Redirect("~/Test/Test_Manage_Package.aspx")

                    End If
                Else
                    MsgBox("ข้อมูลเต็ม")
                End If

            Catch ex As Exception

                Response.Write("Error : " + ex.Message)

            End Try
        Else
            MsgBox("กรุณาเพิ่มข้อมูลขนาดก่อนบันทึก")
            txtPK_WidthTop.Focus()
        End If

    End Sub

    Private Sub ResetAll()

        dt.Clear()
        Response.Redirect("~/Test/Test_Manage_Package.aspx")

    End Sub

    Protected Sub btnPK_Save_Click(sender As Object, e As EventArgs) Handles btnPK_Save.Click

        AddTODB()

    End Sub

    Protected Sub btnPK_Cancel_Click(sender As Object, e As EventArgs) Handles btnPK_Cancel.Click

        ResetAll()

    End Sub

End Class