Modal session related warnings on Mac

Warnings like the one below appear, usually when opening a ComboBox from a modal dialog. Other use-cases may trigger it.

2014-03-19 15:49:14.886 menus[92616:507] modalSession has been exited prematurely - check for a reentrant call to endModalSession:

To reproduce, run the widgets/dialogs/standarddialogs example, select the "Input Dialogs" panel, and then click the "QInputDialog::getItem()" button. After the dialog appears, open the combo box and select a different item. The warning appears while the combo box popup is closing.