Uncaught Exception failing on 6 builders

Builders failed on: 
- KitKat Tablet Tester: 
  https://luci-milo.appspot.com/buildbot/chromium.android/KitKat%20Tablet%20Tester
- Lollipop Phone Tester: 
  https://luci-milo.appspot.com/buildbot/chromium.android/Lollipop%20Phone%20Tester
- Lollipop Tablet Tester: 
  https://luci-milo.appspot.com/buildbot/chromium.android/Lollipop%20Tablet%20Tester
- Marshmallow 64 bit Tester: 
  https://luci-milo.appspot.com/buildbot/chromium.android/Marshmallow%2064%20bit%20Tester
- Marshmallow Tablet Tester: 
  https://luci-milo.appspot.com/buildbot/chromium.android/Marshmallow%20Tablet%20Tester

- Android Tests (dbg)
  https://luci-milo.appspot.com/buildbot/chromium.linux/Android%20Tests%20%28dbg%29


Symptoms are some shards which "had an internal swarming failure", see e.g. "content_browsertests" on:
https://luci-milo.appspot.com/buildbot/chromium.android/Lollipop%20Phone%20Tester/14742

Although the swarming task itself looks fine, e.g.:
https://chromium-swarm.appspot.com/task?id=3854a0e82898a510&refresh=10&show_raw=1

Output ends with:
C  638.048s Main  ********************************************************************************
C  638.048s Main  Summary
C  638.048s Main  ********************************************************************************
C  638.049s Main  [==========] 320 tests ran.
C  638.049s Main  [  PASSED  ] 320 tests.
C  638.049s Main  ********************************************************************************

The traceback on the Uncaught Exception step reads:

Traceback (most recent call last):
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/run.py", line 328, in _new_run
    recipe_result = recipe_script.run(api, properties)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 98, in run
    self.run_steps, properties, self.PROPERTIES, api=api)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 627, in invoke_with_properties
    **additional_args)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 588, in _invoke_with_properties
    return callable_obj(*props, **additional_args)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipes/chromium.py", line 51, in RunSteps
    api.chromium_tests.main_waterfall_steps()
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 745, in _inner
    return func(*a, **kw)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 880, in main_waterfall_steps
    test_runner()
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 316, in test_runner
    t.post_run(self._api_for_tests, suffix)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/chromium_tests/steps.py", line 1853, in post_run
    return self._test.post_run(api, suffix)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/chromium_tests/steps.py", line 1380, in post_run
    super(SwarmingGTestTest, self).post_run(api, suffix)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/chromium_tests/steps.py", line 1258, in post_run
    api.swarming.collect_task(self._tasks[suffix])
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 745, in _inner
    return func(*a, **kw)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/swarming/api.py", line 708, in collect_task
    return task.collect_step(task, **kwargs)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/swarming/api.py", line 507, in <lambda>
    self._gtest_collect_step(test_launcher_summary_output, *args, **kw))
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 745, in _inner
    return func(*a, **kw)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/swarming/api.py", line 911, in _gtest_collect_step
    step_result.presentation)
  File "/b/rr/tmp6_t016/rw/checkout/scripts/slave/recipe_modules/swarming/api.py", line 745, in _display_pending
    if not shard.get('started_ts'):
AttributeError: 'NoneType' object has no attribute 'get'

+martiniss in case you have seen something similar in the past