code: def get_or_add_bgPr(self):
          
          bg = self.bg
          if bg is None or bg.bgPr is None:
              self._change_to_noFill_bg()
          return self.bg.bgPr
docs: 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.