Conncect Database ไม่ได้ค่ะ (Object reference not set to an instance of an object.)

<connectionStrings>
<add name="ConnectionString" connectionString="Data Source=127.0.0.1;Initial Catalog=HelpDesk;Persist Security Info=True;User ID=;Password=" providerName="System.Data.SqlClient">
</add>
</connectionStrings>

Imports Microsoft.VisualBasic
Imports System
Imports System.Data
Imports System.Data.SqlClient
Imports System.Configuration


Public Class clsDatabase
    Inherits System.Web.UI.Page
    Private objConn As SqlConnection
    Private objCmd As SqlCommand
    Private Trans As SqlTransaction
    Private strConnString As String
    Private err As String

    Public Sub New()
        strConnString = System.Configuration.ConfigurationManager.AppSettings("ConnectionString").ToString **error บรรทัดนี้ค่ะ
    End Sub

    Public Function ConnOnly(ByVal xx As String) As DataTable
        Dim dt As New DataTable
        objConn = New SqlConnection
        With objConn
            .ConnectionString = strConnString
            .Open()
        End With
        dt = objConn.GetSchema(xx)
        Return dt
    End Function
End Class