Because, if I get this right it looks something like this:
1.NOTIFICATION_EXIT_TREE or "monitoring" changes to false
2. Area loops through all object that are mapped to be inside it, to call body_exit and body_exit_shape signals
3. If the object already have been deleted/freed this error is displayed
Ok I got it.. I shouldn't freed/destroy my body.object inside the area_exit() method. I will change my algorithm. Thank you very much! :)