[BUG] Unexpected error occurred when worker started before supervisor

[wjsi]

When worker started before supervisor, it produces a ValueError :

Task exception was never retrieved
future: <Task finished name='Task-150' coro=<<coroutine without __name__>()> exception=ValueError('address has to be provided')>
Traceback (most recent call last):
  File "mars/oscar/core.pyx", line 251, in __pyx_actor_method_wrapper
  File "mars/oscar/core.pyx", line 385, in _handle_actor_result
  File "mars/oscar/core.pyx", line 428, in _run_actor_async_generator
  File "mars/oscar/core.pyx", line 429, in mars.oscar.core._BaseActor._run_actor_async_generator
  File "mars/oscar/core.pyx", line 432, in mars.oscar.core._BaseActor._run_actor_async_generator
  File "/home/admin/work/_public-mars-0.9.0b2-py3.8.zip/mars/services/scheduling/worker/workerslot.py", line 247, in upload_slot_usages
    global_resource_ref = await self._get_global_resource_ref()
  File "/home/admin/work/_public-mars-0.9.0b2-py3.8.zip/mars/services/scheduling/worker/workerslot.py", line 134, in _get_global_resource_ref
    [self._global_resource_ref] = await self._cluster_api.get_supervisor_refs(
  File "/home/admin/work/_public-mars-0.9.0b2-py3.8.zip/mars/services/cluster/api/oscar.py", line 117, in get_supervisor_refs
    return await asyncio.gather(
  File "/home/admin/work/_public-mars-0.9.0b2-py3.8.zip/mars/oscar/api.py", line 42, in actor_ref
    return await ctx.actor_ref(*args, **kwargs)
  File "mars/oscar/context.pyx", line 193, in mars.oscar.context.ClientActorContext.actor_ref
  File "mars/oscar/context.pyx", line 162, in mars.oscar.context.ClientActorContext._get_backend_context
ValueError: address has to be provided