code: def get_noneid_references(self):
          
          #return [self.reflst[idx] for idx, idval in enumerate(self) if idval is None]
          try:
              nun = np.array(None).astype(self.dtype)
              return np.array(self.reflst)[self == nun]
          except:
              nun = None
              return np.array(self.reflst)[self is None]
docs: Returns
          -------
          ndarray
          Array of references in self.reflst whose self id is None.