Notification allows TControl to update its PopupMenu property when the pop-up menu it refers to is destroyed.  

Override the Notification method to respond to notifications that other components are about to be destroyed or have just been created. Use the Notification method to update controls that rely on other objects. Remove references to objects that are going away so that the control does not refer to objects that have been destroyed. For example, data-aware objects override the Notification method to update themselves when their data source is removed. Some objects respond to notifications that other objects of a specific type are being created. For example, the session component sets the Session property of new data-aware controls to itself when the AutoSessionName property is true. 

By default, components pass along the notification to their owned components, if any. 

PopupMenu 

Notification