Managed form as child of unmanaged HWND

Question

I need to show my System.Windows.Forms.Form as a child window of an unmanaged C++ HWND. This is the C# SDK code that retrieves the NativeWindow:

public static NativeWindow MainWindow()
{
  Diagnostics.Process process = Diagnostics.Process.GetCurrentProcess();
  if (null == process)
    return null;
  IntPtr handle = process.MainWindowHandle;
  if (IntPtr.Zero == handle)
    return null;

  NativeWindow wnd = new NativeWindow();
  wnd.AssignHandle(handle);

  return wnd;
}

This is how it is implemented in the plug-in:

IWin32Window rh_wnd = Rhino.RhinoApp.MainWindow();
DocEditor.Show(rh_wnd);

This works.... most of the time. But it also fails often the first time I call this code:

Call it again, everything works fine. What's going on?!?