from here: https://build.chromium.org/p/chromium.gpu.fyi/builders/Android%20Release%20%28Nexus%206%29/builds/2775/steps/angle_unittests/logs/stdio

E   43.094s Main  Unrecognized error occurred.
Traceback (most recent call last):
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/test_runner.py", line 949, in main
    return RunTestsCommand(args)
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/test_runner.py", line 769, in RunTestsCommand
    return RunTestsInPlatformMode(args)
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/test_runner.py", line 837, in RunTestsInPlatformMode
    raw_results = test_run.RunTests()
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/pylib/local/device/local_device_test_run.py", line 117, in RunTests
    run_tests_on_device, tc, try_results).pGet(None)
  File "/b/c/b/Android_Release__Nexus_6_/src/third_party/catapult/devil/devil/utils/parallelizer.py", line 236, in pMap
    r.pFinish(None)
  File "/b/c/b/Android_Release__Nexus_6_/src/third_party/catapult/devil/devil/utils/parallelizer.py", line 135, in pFinish
    self._objs.JoinAll()
  File "/b/c/b/Android_Release__Nexus_6_/src/third_party/catapult/devil/devil/utils/reraiser_thread.py", line 186, in JoinAll
    self._JoinAll(watcher, timeout)
  File "/b/c/b/Android_Release__Nexus_6_/src/third_party/catapult/devil/devil/utils/reraiser_thread.py", line 158, in _JoinAll
    thread.ReraiseIfException()
  File "/b/c/b/Android_Release__Nexus_6_/src/third_party/catapult/devil/devil/utils/reraiser_thread.py", line 81, in run
    self._ret = self._func(*self._args, **self._kwargs)
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/pylib/local/device/local_device_environment.py", line 55, in wrapper
    return f(dev, *args, **kwargs)
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/pylib/local/device/local_device_test_run.py", line 67, in run_tests_on_device
    result = self._RunTest(dev, test)
  File "/b/c/b/Android_Release__Nexus_6_/src/build/android/pylib/local/device/local_device_gtest_run.py", line 353, in _RunTest
    suffix='.xml') as device_tmp_results_file:
  File "/b/c/b/Android_Release__Nexus_6_/src/third_party/catapult/devil/devil/android/device_temp_file.py", line 33, in __init__
    self.name = posixpath.join(dir, '%s-%s%s' % (prefix, random_hex, suffix))
  File "/usr/lib/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'

This seemingly happened while a device was dropping offline.