Some examples:
https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?builderName=bob-paladin&buildNumber=4036
https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?builderName=reef-paladin&buildNumber=6779
https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?builderName=scarlet-paladin&buildNumber=4976

Surfaced error is [Test-Logs]: login_VMSanity: FAIL: Unhandled TypeError: 'NoneType' object has no attribute '__getitem__'

Log example:
https://stainless.corp.google.com/browse/chromeos-autotest-results/240140782-chromeos-test/
...
09/20 21:04:51.420 INFO |           browser:0221| Browser is closed.
09/20 21:04:51.600 WARNI|              test:0606| The test failed with the following exception
Traceback (most recent call last):
  File "/usr/local/autotest/common_lib/test.py", line 600, in _exec
    _call_test_function(self.execute, *p_args, **p_dargs)
  File "/usr/local/autotest/common_lib/test.py", line 806, in _call_test_function
    raise error.UnhandledTestFail(e)
UnhandledTestFail: Unhandled TypeError: 'NoneType' object has no attribute '__getitem__'
Traceback (most recent call last):
  File "/usr/local/autotest/common_lib/test.py", line 800, in _call_test_function
    return func(*args, **dargs)
  File "/usr/local/autotest/common_lib/test.py", line 464, in execute
    postprocess_profiled_run, args, dargs)
  File "/usr/local/autotest/common_lib/test.py", line 371, in _call_run_once
    self.run_once(*args, **dargs)
  File "/usr/local/autotest/tests/login_VMSanity/login_VMSanity.py", line 13, in run_once
    vm_sanity.VMSanity().Run()
  File "/usr/local/autotest/bin/vm_sanity.py", line 55, in Run
    self.RunScreenlockTest()
  File "/usr/local/autotest/bin/vm_sanity.py", line 122, in RunScreenlockTest
    exception=TestFail('Screen not locked'))
  File "/usr/local/autotest/common_lib/utils.py", line 2756, in poll_for_condition
    value = condition()
  File "/usr/local/autotest/bin/vm_sanity.py", line 120, in <lambda>
    lambda: cr.login_status['isScreenLocked'],
TypeError: 'NoneType' object has no attribute '__getitem__'
...

Suspected CL is https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1236653