AppHost::_WindowActivated reads/writes various fields in a thread unsafe manner - maybe extra bad for ARM

Additionally, that function might be called through the destructor when XAML empties the remaining Win32 message queue and we get called through `IslandWindow::MessageHandler`. `_window` is set to `nullptr` before that happens and we should check it before it's used through `_CurrentDesktopGuid`.

Start at AppHost::_WindowActivated and trace the destructor path through IslandWindow::MessageHandler. Inspect the thread-unsafe field accesses, especially _window and _CurrentDesktopGuid; done means the accesses are safe and the null _window case cannot be dereferenced.

Written by the indexing model from the issue text.