title: Python gprc AttributeError: 'NoneType' object has no attribute 'message_types_by_name'
body: <p>I just want to complete a simple demo, but I got this error message.</p>
  <pre class="lang-none prettyprint-override"><code>Traceback (most recent call last):
    File &quot;E:/grpc/biliService.py&quot;, line 10, in &lt;module&gt;
      import hello_bilibili_pb2 as pb2
    File &quot;E:\grpc\hello_bilibili_pb2.py&quot;, line 21, in &lt;module&gt;
      _HELLODEWEIREQ = DESCRIPTOR.message_types_by_name['HelloDeweiReq']
  AttributeError: 'NoneType' object has no attribute 'message_types_by_name'
  </code></pre>