core: Google Calendar integration stopped working

Home Assistant release with the issue:

0.99.3

Last working Home Assistant release (if known): 0.98.x

Operating environment (Hass.io/Docker/Windows/etc.):

Docker

Integration:

https://www.home-assistant.io/integrations/calendar.google/

Description of problem: My Google calendars are no longer appearing. Logs show the following 2 errors:

Error executing service <ServiceCall google.scan_for_calendars (c:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)>

Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/core.py", line 1242, in _safe_execute
    await self._execute_service(handler, service_call)
  File "/usr/src/homeassistant/homeassistant/core.py", line 1261, in _execute_service
    await self._hass.async_add_executor_job(handler.func, service_call)
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google/__init__.py", line 255, in _scan_for_calendars
    service = calendar_service.get()
  File "/usr/src/homeassistant/homeassistant/components/google/__init__.py", line 349, in get
    http = credentials.authorize(httplib2.Http())
AttributeError: 'NoneType' object has no attribute 'authorize'

google: Error on device update!

Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 292, in _async_add_entity
    await entity.async_device_update(warning=False)
  File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 441, in async_device_update
    await self.hass.async_add_executor_job(self.update)
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google/calendar.py", line 104, in update
    self.data.update()
  File "/usr/src/homeassistant/homeassistant/util/__init__.py", line 240, in wrapper
    result = method(*args, **kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google/calendar.py", line 170, in update
    service, params = self._prepare_query()
  File "/usr/src/homeassistant/homeassistant/components/google/calendar.py", line 133, in _prepare_query
    service = self.calendar_service.get()
  File "/usr/src/homeassistant/homeassistant/components/google/__init__.py", line 349, in get
    http = credentials.authorize(httplib2.Http())
AttributeError: 'NoneType' object has no attribute 'authorize'