code: def GetBlockHash(self, height):
          
          if self._current_block_height < height:
              return
  
          if len(self._header_index) <= height:
              return
  
          return self._header_index[height]
docs: Get the block hash by its block height
          Args:
              height(int): height of the block to retrieve hash from.
  
          Returns:
              bytes: a non-raw block hash (e.g. b'6dd83ed8a3fc02e322f91f30431bf3662a8c8e8ebe976c3565f0d21c70620991', but not b'\x6d\xd8...etc'