进行截图时异常

**描述问题bug**:
我使用 ` b64img, fmt = poco.snapshot(width=screen_size[1]) `进行截图时, 出现 hrpc.exceptions.RpcRemoteException 异常, 异常信息如下:
```
File "/usr/local/python3/lib/python3.7/site-packages/flybirds/core/plugin/plugins/default/screen.py", line 31, in screen_shot
b64img, fmt = poco.snapshot(width=screen_size[1])
File "/usr/local/python3/lib/python3.7/site-packages/poco/pocofw.py", line 433, in snapshot
return self.agent.screen.getScreen(width)
File "/usr/local/python3/lib/python3.7/site-packages/poco/drivers/android/uiautomation.py", line 75, in getScreen
b64img = self.screen.getScreen(width)
File "/usr/local/python3/lib/python3.7/site-packages/hrpc-1.0.9-py3.7.egg/hrpc/object_proxy.py", line 92, in __call__
return self._client__.evaluate(self.__call_no_evaluate__(remote_obj_cache, *args))
File "/usr/local/python3/lib/python3.7/site-packages/hrpc-1.0.9-py3.7.egg/hrpc/client.py", line 81, in evaluate
raise RpcRemoteException(resp)
hrpc.exceptions.RpcRemoteException: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.Bitmap.getHeight()' on a null object reference

|-- Remote Traceback --|

java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.Bitmap.getHeight()' on a null object reference
at com.netease.open.pocoservice.Screen.getScreen(Screen.java:40)
at java.lang.reflect.Method.invoke(Native Method)
at com.netease.open.hrpc.backend.RpcServer.onRequest(RpcServer.java:171)
at com.netease.open.hrpc.backend.RpcServer.serve(RpcServer.java:57)
at fi.iki.elonen.NanoHTTPD$HTTPSession.execute(NanoHTTPD.java:840)
at fi.iki.elonen.NanoHTTPD$ClientHandler.run(NanoHTTPD.java:189)
at java.lang.Thread.run(Thread.java:920)

|-- Remote Traceback end --|
```

**python 版本**: `python3.7`

**pocoui版本:** ` 1.0.85 `

**hrpc 版本**: ` 1.0.9 `

**设备**:

- 型号: [谷歌原生模拟器]
- 系统: [ANDROID26(8.0)]