New Post: Can't Open .dspx File

I use this code in toolstripmenuitem

Private Sub BukaProyekToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles BukaProyekToolStripMenuItem.Click
        Using OpenDlg = New OpenFileDialog()
            OpenDlg.Filter = AppManager1.SerializationManager.OpenDialogFilterText
            OpenDlg.Title = "Open File"
            OpenDlg.RestoreDirectory = True

            If OpenDlg.ShowDialog() <> Windows.Forms.DialogResult.OK Then
                Return
            End If

            Try
                Me.Cursor = Cursors.WaitCursor
                AppManager1.SerializationManager.OpenProject(OpenDlg.FileName)
                AppManager1.Map.Invalidate()
                Me.Cursor = Cursors.Default
            Catch ex As Exception
                MsgBox(Err.Description, vbCritical + vbOKOnly, Err.Number)
            End Try
        End Using
    End Sub

I can open file explorer box, but when I click "Open", i get this message in dialog box
"Object references not set to an instance of an object"

Anybody help?

Regards,
Rizky