AutoCAD Customization: .NET: Turn on/off Viewport Error

Hi All,

Please let me know this error:

When I try the code:

Public Sub HVP()
        Dim doc As Document = Application.DocumentManager.MdiActiveDocument
        Dim ed As Editor = doc.Editor
        Dim db As Database = HostApplicationServices.WorkingDatabase
        Using DocLock As DocumentLock = Application.DocumentManager.MdiActiveDocument.LockDocument()
            Using tr As Transaction = db.TransactionManager.StartTransaction()
                Try
                    Dim layoutdict As DBDictionary = TryCast(tr.GetObject(db.LayoutDictionaryId, OpenMode.ForRead, False, True), DBDictionary)
                    For Each entry As DBDictionaryEntry In layoutdict
                        If entry.Key <> "Model" Then
                            Dim lay As Layout = TryCast(tr.GetObject(entry.Value, OpenMode.ForRead, False, True), Layout)
                            For Each vpid As ObjectId In lay.GetViewports()
                                Dim vp As Autodesk.AutoCAD.DatabaseServices.Viewport = TryCast(tr.GetObject(vpid, OpenMode.ForWrite, True, True), Autodesk.AutoCAD.DatabaseServices.Viewport)

                                    vp.On = False

                            Next
                        End If
                    Next
                Catch ex As Exception
                    MsgBox(ex.Message & ex.StackTrace & ex.ErrorStatus)
                End Try
                tr.Commit()
            End Using
        End Using

Thanks for any help.