opentelemetry-python: AttributeError: 'NoneType' object has no attribute 'get_current'

Describe your environment runtime: python3.6.1; sdk version: 1.7.1

Steps to reproduce Describe exactly how to reproduce the error. Include a code sample if applicable.

What is the expected behavior? What did you expect to see? I use python3.6.1 runtime; But i have a problem，

File "index.py", line 17, in <module>
with tracer.start_as_current_span("foo"):
File "/opt/python3.6.1/lib/python3.6/contextlib.py", line 82, in enter
return next(self.gen)
File "/opt/python3.6.1/lib/python3.6/site-packages/opentelemetry/sdk/trace/init.py", line 980, in start_as_current_span
set_status_on_exception=set_status_on_exception,
File "/opt/python3.6.1/lib/python3.6/site-packages/opentelemetry/sdk/trace/init.py", line 1003, in start_span
context
File "/opt/python3.6.1/lib/python3.6/site-packages/opentelemetry/trace/propagation/init.py", line 48, in get_current_span
span = get_value(_SPAN_KEY, context=context)
File "/opt/python3.6.1/lib/python3.6/site-packages/opentelemetry/context/init.py", line 96, in get_value
return context.get(key) if context is not None else get_current().get(key)
File "/opt/python3.6.1/lib/python3.6/site-packages/opentelemetry/context/init.py", line 67, in wrapper
return func(args, *kwargs) # type: ignore[misc]
File "/opt/python3.6.1/lib/python3.6/site-packages/opentelemetry/context/init.py", line 131, in get_current
return _RUNTIME_CONTEXT.get_current() # type:ignore
AttributeError: 'NoneType' object has no attribute 'get_current'

my code is:

from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import (
BatchSpanProcessor,
ConsoleSpanExporter,
)

provider = TracerProvider()
processor = BatchSpanProcessor(ConsoleSpanExporter())
provider.add_span_processor(processor)
trace.set_tracer_provider(provider)

tracer = trace.get_tracer(name)

with tracer.start_as_current_span("foo"):
with tracer.start_as_current_span("bar"):
with tracer.start_as_current_span("baz"):
print("Hello world from OpenTelemetry Python!")

What is the actual behavior? What did you see instead?

Additional context Add any other context about the problem here. https://opentelemetry-python.readthedocs.io/en/stable/getting-started.html