One issue we often see with complex object interdependencies is objects that relay on other objects in order to complete their "On_delete" logic.
When you exit PxPlus with objects still active, the system attempts to close all the objects however it has no idea of the order your application may require in order to shutdown properly. Internally it attempts to delete objects starting from the most recently created ones first based on their reference count but that may not always work or be viable.
We have seen, sadly on numerous occasions, objects that when being deleted access resources on other objects without regard as to whether the other object still exists. This often results in some form of error trap which in itself may reference objects that are no longer present.
Care should be taken to always check in that any object deletion logic that relies of external objects first checks that the object is still present and handles it appropriately.