运行截图时报错

```
INTERNALERROR> File "/Users/test/Project_pytest/new_auto_test/test-app-autotest/conftest.py", line 88, in capture_screenshot
INTERNALERROR> screenshot, fmt = poco.snapshot(width=720)
INTERNALERROR> File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/poco/pocofw.py", line 433, in snapshot
INTERNALERROR> return self.agent.screen.getScreen(width)
INTERNALERROR> File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/poco/drivers/android/uiautomation.py", line 73, in getScreen
INTERNALERROR> b64img = self.screen.getScreen(width)
INTERNALERROR> File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/hrpc/object_proxy.py", line 92, in __call__
INTERNALERROR> return self._client__.evaluate(self.__call_no_evaluate__(remote_obj_cache, *args))
INTERNALERROR> File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/hrpc/client.py", line 81, in evaluate
INTERNALERROR> raise RpcRemoteException(resp)
INTERNALERROR> hrpc.exceptions.RpcRemoteException: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.Bitmap.getHeight()' on a null object reference
```
hrpc.exceptions.RpcRemoteException 异常，并且又涉及到了```android.graphics.Bitmap.getHeight()```错误，希望解决。