The destructor is called, however all windows I show shares the same element of the main dialog in the task bar.

There another way to close all windows and to call their destructors?
Or as alternative solution, show this other dialogs without sharing the same element in the task bar?
If your widgets are properly parented, I dont see need to even handle closeEvent. From what you posted it seems you want to call the destructor of custom widgets before the application or main window closes.
Am I right in understanding your requirement ?