Before the window is shown, the default reaction to the back key is to call QCoreApplication::quit(). There is no way for the user to override this, so if they do not want the application to quit when the user hits the back key, they have no way of achieving that. In addition, there are crashes triggered when quit() is called too early, which is a separate but related issue.

Would it perhaps be better to just pass back key events to the application when there is no window showing?