What is wrong with this code

err. occurs An attempt has been made to free an RCW that is in use

VS 2010 on Win 7 64 bit (runs without any problem on 32 bit)

Err. occurs while loading a form containing CrystalReportViewer

Code:

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        Dim closeapp As Boolean = False

        Do While closeapp = False
            If OpenWindow = NewWindow.SelUser Then
                OpenWindow = NewWindow.MainMenu
                SetUser()
            ElseIf OpenWindow = NewWindow.MainMenu Then
                OpenWindow = NewWindow.End
                MainMenu.ShowDialog()
                MainMenu.Dispose()
            Else : closeapp = True
            End If
        Loop

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MainMenu is MDI Form which contains many child form

Please Help
Amit Saraf