An event handler attached to Window::EventDestructionStarted could potentially created new Window objects. This is an issue if this event is triggered due to the global system cleanup that occurs in CEGUI::System::System - we could end up with resource leaks or even a deadlock situation.
While it would be possible to inhibit events from firing at system shutdown, there are many valid uses for these events (such as additional resource clean-up).
I propose to add the ability to 'lock' the window manager during system shutdown so that any attempt to create a window during this period would result in an InvalidRequestException.
Reproducibility: have not tried