title: Beam: AfterProcessingTime cause 'NoneType' object has no attribute 'time'
body: <p>With the following codes under beam <code>2.14.0</code></p>
  
  <pre class="lang-py prettyprint-override"><code>| "FixedWindow" &gt;&gt; beam.WindowInto(beam.window.FixedWindows(4 * 60),
                        trigger=beam.trigger.Repeatedly(
                                beam.trigger.AfterProcessingTime(delay=1 * 60)
                            ),
                        accumulation_mode=beam.trigger.AccumulationMode.DISCARDING)
  </code></pre>
  
  <p>The following error comes up</p>
  
  <pre><code>Traceback (most recent call last):
    File "beam_home.py", line 287, in &lt;module&gt;
      run()
    File "beam_home.py", line 282, in run
      p.run().wait_until_finish()
    File "/usr/local/lib/python2.7/site-packages/apache_beam/pipeline.py", line 406, in run
      self._options).run(False)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/pipeline.py", line 419, in run
      return self.runner.run_pipeline(self, self._options)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/direct/direct_runner.py", line 128, in run_pipeline
      return runner.run_pipeline(pipeline, options)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 294, in run_pipeline
      default_environment=self._default_environment))
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 301, in run_via_runner_api
      return self.run_stages(stage_context, stages)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 383, in run_stages
      stage_context.safe_coders)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 655, in _run_stage
      result, splits = bundle_manager.process_bundle(data_input, data_output)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 1460, in process_bundle
      process_bundle_id, transform_id, elements)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 1356, in _send_input_to_worker
      for byte_stream in byte_streams:
    File "/usr/local/lib/python2.7/site-packages/apache_beam/runners/portability/fn_api_runner.py", line 186, in __iter__
      for wkvs in windowed_key_values(key, windowed_values):
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 966, in process_entire_key
      state, windowed_values, output_watermark):
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 1130, in process_elements
      self.trigger_fn.on_element(value, window, context)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 515, in on_element
      self.underlying.on_element(element, window, context)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 373, in on_element
      self.early.on_element(element, window, NestedContext(context, 'early'))
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 515, in on_element
      self.underlying.on_element(element, window, context)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 307, in on_element
      '', TimeDomain.REAL_TIME, context.get_current_time() + self.delay)
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 759, in get_current_time
      return self._outer.get_current_time()
    File "/usr/local/lib/python2.7/site-packages/apache_beam/transforms/trigger.py", line 733, in get_current_time
      return self._clock.time()
  AttributeError: 'NoneType' object has no attribute 'time'
  </code></pre>
  
  <p>Anything am I missing?</p>