Why should a removeSubWindow() raise exception?

[Raccoon29]

Hi all,
my question tells for your very wide experience:
what are all the motivations for which a QMdiArea::removeSubWindow(QWidget* widget) should fail, raising an exception (and, so, crashing the program)?

Details:
MDI application.
QDialog as widget in a QMdiSubWindow.
When closing it, following rows are executed:

mdiArea->removeSubWindow(wArticle);
delete pArticle;
pArticle=NULL;
wArticle=NULL;

mdiArea->removeSubWindow(wArticle);
delete pArticle;
pArticle=NULL;
wArticle=NULL;

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where:
mdiArea => QMdiArea*
wArticle => QMdiSubWindow*
pArticle => QDialog* (contained by wArticle)
The first of the four instructions raises an exception and makes crash the program.
So, what I'm asking is: what has wArticle to have of strange (since I checked it is not NULL) to make the method fail?