pocoservice 进行截图时失败

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**(重要！问题分类)**
* 图像识别相关问题 -> https://github.com/AirtestProject/Airtest/issues
* 测试开发环境AirtestIDE使用问题 -> https://github.com/AirtestProject/AirtestIDE/issues
* 控件识别、树状结构、poco库报错 -> 此仓库

**描述问题bug**
我在代码里使用 `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:

```
2022-04-12 20:03:22,766 - flybirds_log - WARNING - Traceback (most recent call last):
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 --|

```

**复现步骤**
```
from poco.drivers.android.uiautomation import AndroidUiautomationPoco

poco = AndroidUiautomationPoco(
use_airtest_input=True, screenshot_each_action=False
)
screen_size=[1080, 1920]
b64img, fmt = poco.snapshot(width=screen_size[1])

```
**预期效果**
截图成功

**相关截图**
(贴出遇到问题时的截图内容，如果有的话)

**python 版本:** `python3.7`

**poco 版本:** `1.0.85`
> poco版本通过`pip freeze`可以命令可以查到

**设备:**
- 型号: [谷歌原生模拟器]
- 系统: [ANDROID26(8.0)]
- (别的信息)

**其他相关环境信息**
(其他运行环境，例如在linux ubuntu16.04上运行异常，在windows上正常。)