Ссылка на объект не установлена на экземпляр объекта. Эта ошибка приходит в vb.... помогите мне, пожалуйста

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Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 


Line 33:     End Sub
Line 34:     Private Sub BindGrid()
Line 35:         Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
Line 36:         Using con As New SqlConnection(strConnString)
Line 37:             Using cmd As New SqlCommand()

Source File: E:\LanguageLab\admin\listeningAplay.aspx.vb    Line: 35 

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.]
   admin_listeningAplay.BindGrid() in E:\LanguageLab\admin\listeningAplay.aspx.vb:35
   admin_listeningAplay.Page_Load(Object sender, EventArgs e) in E:\LanguageLab\admin\listeningAplay.aspx.vb:31
   System.Web.UI.Control.OnLoad(EventArgs e) +95
   System.Web.UI.Control.LoadRecursive() +59
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +678

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.1637.0

Что я уже пробовал:

Imports System
Imports System.Web
Imports System.Data.SqlClient
Imports System.Configuration
Imports System.IO

Partial Class admin_listeningAplay
    Inherits Page

    Protected Sub btnUpload_Click(sender As Object, e As EventArgs) Handles btnUpload.Click
        Using br As New BinaryReader(FileUpload1.PostedFile.InputStream)
            Dim bytes As Byte() = br.ReadBytes(CInt(FileUpload1.PostedFile.InputStream.Length))
            Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
            Using con As New SqlConnection(strConnString)
                Using cmd As New SqlCommand()
                    cmd.CommandText = "insert into Audio(Name, ContentType, Data) values (@Name, @ContentType, @Data)"
                    cmd.Parameters.AddWithValue("@Name", Path.GetFileName(FileUpload1.PostedFile.FileName))
                    cmd.Parameters.AddWithValue("@ContentType", "audio/mpeg3")
                    cmd.Parameters.AddWithValue("@Data", bytes)
                    cmd.Connection = con
                    con.Open()
                    cmd.ExecuteNonQuery()
                    con.Close()
                End Using
            End Using
        End Using
        Response.Redirect(Request.Url.AbsoluteUri)
    End Sub
    Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
        If Not IsPostBack Then
            BindGrid()
        End If
    End Sub
    Private Sub BindGrid()
        Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Using con As New SqlConnection(strConnString)
            Using cmd As New SqlCommand()
                cmd.CommandText = "select Id, Name from tblFiles"
                cmd.Connection = con
                con.Open()
                GridView1.DataSource = cmd.ExecuteReader()
                GridView1.DataBind()
                con.Close()
            End Using
        End Using
    End Sub

End Class

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