error while merging summaries using tf.compat.v1.summary.merge_all()

Question

For some reason, self._summaries object is None.

This results in following error:

  File "main.py", line 216, in <module>
    tf.compat.v1.app.run()
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/platform/app.py", line 40, in run
    _run(main=main, argv=argv, flags_parser=_parse_flags_tolerate_undef)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/absl/app.py", line 312, in run
    _run_main(main, args)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/absl/app.py", line 258, in _run_main
    sys.exit(main(argv))
  File "main.py", line 203, in main
    setup_training(hps.mode, generator, discriminator, generator_batcher, discriminator_batcher, generator_val_batcher, discriminator_val_batcher)
  File "main.py", line 160, in setup_training
    trainer.adversarial_train(generator, discriminator, generator_batcher, discriminator_batcher, generator_val_batcher, discriminator_val_batcher, summary_writer, sess_context_manager)
  File "/home/20bce120/tf2/trainer.py", line 106, in adversarial_train
    result_train = generator.run_train_step(sess, batch)
  File "/home/20bce120/tf2/generator.py", line 636, in run_train_step
    return sess.run(to_return, feed_dict)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 968, in run
    run_metadata_ptr)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 1176, in _run
    self._graph, fetches, feed_dict_tensor, feed_handles=feed_handles)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 487, in __init__
    self._fetch_mapper = _FetchMapper.for_fetch(fetches)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 270, in for_fetch
    return _DictFetchMapper(fetch)
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 419, in __init__
    _FetchMapper.for_fetch(fetch) for fetch in fetches.values()
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 419, in <listcomp>
    _FetchMapper.for_fetch(fetch) for fetch in fetches.values()
  File "/apps/codes/anaconda3/envs/myenv2/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 265, in for_fetch
    (fetch, type(fetch)))
TypeError: Fetch argument None has invalid type <class 'NoneType'>

Code :

    def build_graph(self):
        
        tf.logging.info('Building graph...')
        t0 = time.time()
        self._add_placeholders()
        self._add_seq2seq()
        self.global_step = tf.Variable(0, name='global_step', trainable=False)
        if self._hps.mode == 'train'or self._hps.mode == 'pretrain':
            self._add_train_op()
            self._rollout()
            
        self._summaries = tf.compat.v1.summary.merge_all()
        tf.logging.info('Time to build graph: %i seconds', time.time() - t0)

I tried adding one tf.scalar_summary() before calling tf.compat.v1.summary.merge_all() but it didn't work.