GetWindowHandle15 throws InvalidComObjectException

In a ribbon handler, I would like to set the owner of my dialog to the Excel window handle. I think I'm doing this right, here's my entire helper function to show a dialog (F#):

```
let showDialog (window: Window) =
let helper = WindowInteropHelper window
helper.Owner <- ExcelDnaUtil.WindowHandle
window.ShowInTaskbar <- false
window.ShowDialog().Value
```

This works fine on my development machine (Windows 7 + Office 2010), but on at least one customer machine, accessing the WindowHandle property throws an exception:

> System.Runtime.InteropServices.InvalidComObjectException: COM object that has been separated from its underlying RCW cannot be used.
at System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters)
at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)
at ExcelDna.Integration.ExcelDnaUtil.GetWindowHandle15()
at Utils.showDialog(Window window)

Looking at the code for GetWindowHandle15(), I see it's a bit tricky to correctly get the Excel window handle (thanks, Microsoft), and if it fails, it throws InvalidOperationException - presumably in that case it really is an invalid operation. InvalidComObjectException here seems to be completely unexpected.