Adds a custom button to the button box for this popup widget.
        
        :param      button | <QAbstractButton> || <str>
        
        :return     <button> || None (based on if a button or string was given)

def addButton(self, button, role=QDialogButtonBox.ActionRole):
        """
        Adds a custom button to the button box for this popup widget.
        
        :param      button | <QAbstractButton> || <str>
        
        :return     <button> || None (based on if a button or string was given)
        """
        return self._buttonBox.addButton(button, role)