Return `p:bg/p:bgPr` grandchild.

        If no such grandchild is present, any existing `p:bg` child is first
        removed and a new default `p:bg` with noFill settings is added.

def get_or_add_bgPr(self):
        """Return `p:bg/p:bgPr` grandchild.

        If no such grandchild is present, any existing `p:bg` child is first
        removed and a new default `p:bg` with noFill settings is added.
        """
        bg = self.bg
        if bg is None or bg.bgPr is None:
            self._change_to_noFill_bg()
        return self.bg.bgPr