Running step decorated function outside tests causes Exception

[Sup3rGeo]

I'm submitting a ...

• bug report
• feature request
• support request => Please do not submit support request here, see note at the top of this template.

What is the current behavior?

Tests break when Running step decorated function outside test or fixture

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

Running this simple test shows the error:

import allure

with allure.step("step outside"):
    pass

def test1():
    assert 1 == 1

________________________ ERROR collecting test_one.py _________________________
test_one.py:7: in <module>
    with allure.step("step outside"):
d:\work\allure-python\allure-python-commons\allure_commons\_allure.py:131: in __enter__
    plugin_manager.hook.start_step(uuid=self.uuid, title=self.title, params=params)
c:\python36\lib\site-packages\pluggy\__init__.py:617: in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
c:\python36\lib\site-packages\pluggy\__init__.py:222: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
c:\python36\lib\site-packages\pluggy\__init__.py:216: in <lambda>
    firstresult=hook.spec_opts.get('firstresult'),
d:\work\allure-python\allure-pytest\allure_pytest\listener.py:38: in start_step
    self.allure_logger.start_step(None, uuid, step)
d:\work\allure-python\allure-python-commons\allure_commons\reporter.py:77: in start_step
    self._items[parent_uuid].steps.append(step)
E   KeyError: None
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!

What is the expected behavior?

Simply ignore the allure step.

What is the motivation / use case for changing the behavior?

Sometimes we can decorate a function as step and this function is used outside a test function or fixture. This should not break the tests.

Please tell us about your environment:

• Allure version: 2.6.0
• Test framework: pytest@3.6
• Allure adaptor: allure-pytest@2.3.2b1

Other information