A Boolean value that indicates whether the window prevents application termination when modal.
The value of this property is YES if the window prevents application termination when modal; otherwise, NO. The default value is YES.
Usually, application termination is prevented when a modal window or sheet is open, without consulting the application delegate. Some windows may wish not to prevent termination, however. Setting this property to NO overrides the default behavior and allows termination to proceed even if the window is open, either through the sudden termination path if enabled, or after consulting the application delegate.