Returns the list of top level nodes for this tree.
        
        :return     [<QtGui.QTreeWidgetItem>, ..]

def topLevelItems(self):
        """
        Returns the list of top level nodes for this tree.
        
        :return     [<QtGui.QTreeWidgetItem>, ..]
        """
        try:
            for i in xrange(self.topLevelItemCount()):
                yield self.topLevelItem(i)
        except RuntimeError:
            # can be raised when iterating on a deleted tree widget.
            return