Return the notes master placeholder this notes slide placeholder
        inherits from, or |None| if no placeholder of the matching type is
        present.

def _base_placeholder(self):
        """
        Return the notes master placeholder this notes slide placeholder
        inherits from, or |None| if no placeholder of the matching type is
        present.
        """
        notes_master = self.part.notes_master
        ph_type = self.element.ph_type
        return notes_master.placeholders.get(ph_type=ph_type)