code: def _get_mixer(self):
      if self._mixer is None:
          raise RuntimeError(
              "mixer not initialized, run dh_exchange first"
          )
      return self._mixer
  
file_path: FATE/python/fate/arch/protocol/secure_aggregation/_secure_aggregation.py
imports: import typing
  import numpy
  from fate.arch import Context
  from fate.arch.protocol.diffie_hellman import DiffieHellman
  from fate_utils.secure_aggregation_helper import MixAggregate, RandomMix
label_desc: Use image node_id 2 for calling the SecureAggregatorClient obj's underlying member method code with example usage: obj._get_mixer() and returns: self