[BUG] Async 'NoneType' object has no attribute 'result'

[del-zhenwu]

Describe the bug

____________________________________________________________________________________________________ TestFlushAsync.test_flush_async _____________________________________________________________________________________________________

self = <test_flush.TestFlushAsync object at 0x7fc365a710f0>, connect = <milvus.client.stub.Milvus object at 0x7fc365a71ac8>, collection = 'test_flush_WFrtmIBT'

    def test_flush_async(self, connect, collection):
        nb = 100000
        vectors = gen_vectors(nb, dim)
        connect.add_vectors(collection, vectors)
        logging.getLogger().info("before")
        future = connect.flush([collection], _async=True, _callback=self.check_status)
        logging.getLogger().info("after")
        future.done()
>       status = future.result()

test_flush.py:270: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <milvus.client.asynch.FlushFuture object at 0x7fc3655e2da0>, kwargs = {}, to = None

    def result(self, **kwargs):
        self.exception()
        with self._condition:
            # future not finished. wait callback being called.
            to = kwargs.get("timeout", None)
>           self._response = self._future.result(timeout=to)
E           AttributeError: 'NoneType' object has no attribute 'result'

/usr/local/lib/python3.6/site-packages/milvus/client/asynch.py:94: AttributeError

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Environment details

• Hardware/Softward conditions (OS, CPU, GPU, Memory)
• Method of installation (Docker, or from source)
• Milvus version (v0.3.1, or v0.4.0)
• Milvus configuration (Settings you made in server_config.yaml)

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