After this waterfall restart https://chrome-internal.googlesource.com/infradata/master-manager/+/918e2dcb7612f09b23e958fa51324e61f150ccc3

Every bot in the GPU and GPU FYI waterfalls fails with this following Python exception:

(view as text)
Traceback (most recent call last):
  File "/home/chrome-bot/buildbot/build/third_party/buildbot_8_4p1/buildbot/process/buildstep.py", line 810, in _startStep_2
    doStep.addCallback(self._startStep_3)
  File "/home/chrome-bot/buildbot/build/third_party/twisted_10_2/twisted/internet/defer.py", line 301, in addCallback
    callbackKeywords=kw)
  File "/home/chrome-bot/buildbot/build/third_party/twisted_10_2/twisted/internet/defer.py", line 290, in addCallbacks
    self._runCallbacks()
  File "/home/chrome-bot/buildbot/build/third_party/twisted_10_2/twisted/internet/defer.py", line 547, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
--- <exception caught here> ---
  File "/home/chrome-bot/buildbot/build/third_party/buildbot_8_4p1/buildbot/process/buildstep.py", line 816, in _startStep_3
    if self.start() == SKIPPED:
  File "/home/chrome-bot/buildbot/build/scripts/master/chromium_step.py", line 370, in start
    shell.ShellCommand.start(self)
  File "/home/chrome-bot/buildbot/build/third_party/buildbot_8_4p1/buildbot/steps/shell.py", line 243, in start
    self._flattenList(tmp, self.command)
  File "/home/chrome-bot/buildbot/build/third_party/buildbot_8_4p1/buildbot/steps/shell.py", line 230, in _flattenList
    self._flattenList(mainlist, x)
  File "/home/chrome-bot/buildbot/build/third_party/buildbot_8_4p1/buildbot/steps/shell.py", line 226, in _flattenList
    for x in commands:
exceptions.TypeError: 'NoneType' object is not iterable

From https://build.chromium.org/p/chromium.gpu.fyi/builders/GPU%20Win%20Builder/builds/74978/steps/steps/logs/err.text

Example build: https://build.chromium.org/p/chromium.gpu.fyi/builders/GPU%20Win%20Builder/builds/74978/