I am getting an error when trying to download file from database.

I get this error when trying to download the file

Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Modules.ID_LIT.View.GetData(SqlCommand cmd) at DotNetNuke.Modules.ID_LIT.View.Button4_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---

I am using this example from your site:

http://www.aspsnippets.com/Articles/Save-and-Retrieve-Files-from-SQL-Server-Database-using-ASP.Net.aspx

here is my code:

    Public Function GetData(ByVal cmd As SqlCommand) As DataTable
        Dim dt As New DataTable
        Dim strConnString As String = System.Configuration.ConfigurationManager.ConnectionStrings("mystring").ConnectionString
        Dim con As New SqlConnection(strConnString)
        Dim sda As New SqlDataAdapter
        cmd.CommandType = CommandType.Text
        cmd.Connection = con
        Try
            con.Open()
            sda.SelectCommand = cmd
            sda.Fill(dt)
            Return dt
        Catch ex As Exception
            Response.Write(ex.Message)
            Return Nothing
        Finally
            con.Close()
            sda.Dispose()
            con.Dispose()
        End Try
    End Function

    Protected Sub Button4_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button4.Click
        Dim litid As Label = CType(FormView2.FindControl("LITIDLabel"), Label)
        Dim strQuery As String = "SELECT LITName, LITObject FROM LITLiterature WHERE LITID = @LITID"
        Dim cmd As SqlCommand = New SqlCommand(strQuery)
        cmd.Parameters.Add("@LITID", SqlDbType.Int).Value = litid.text
        Dim dt As DataTable = GetData(cmd)
        If dt IsNot Nothing Then
            download(dt)
        End If
    End Sub

    Protected Sub download(ByVal dt As DataTable)
        Dim bytes() As Byte = CType(dt.Rows(0)("LITObject"), Byte())
        Response.Buffer = True
        Response.Charset = ""
        Response.Cache.SetCacheability(HttpCacheability.NoCache)
        Response.ContentType = dt.Rows(0)("ContentType").ToString()
        Response.AddHeader("content-disposition", "attachment;filename=" & dt.Rows(0)("LITName").ToString())
        Response.BinaryWrite(bytes)
        Response.Flush()
        Response.End()
    End Sub