使用multiprocessing报错，AttributeError: 'NoneType' object has no attribute 'snapshot'

## 需求：
一边执行基本的识别点击操作(main_operation())，一边检测有没有对应的其他图片出现在界面中(sub_operation())，所以使用multiprocessing来处理。

## 问题：
不使用multiprocessing只执行上述一个操作时正常，但使用multiprocessing之后，哪怕只执行(main_operation())也会出现如下错误
```
Process Process-1:
Traceback (most recent call last):
File "C:\Users\a\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\process.py", line 314, in _bootstrap
self.run()
File "C:\Users\a\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\process.py", line 108, in run
self._target(*self._args, **self._kwargs)
File "H:\code\python3\python\QuickTools\AirtestTest.py", line 238, in main_operation
all_pos = find_all(Template(r"resource/safe_state.PNG"))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\a\AppData\Local\Programs\Python\Python311\Lib\site-packages\airtest\utils\logwraper.py", line 122, in wrapper
res = f(*args, **kwargs)
^^^^^^^^^^^^^^^^^^
File "C:\Users\a\AppData\Local\Programs\Python\Python311\Lib\site-packages\airtest\core\api.py", line 655, in find_all
screen = G.DEVICE.snapshot(quality=ST.SNAPSHOT_QUALITY)
^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'snapshot'
```

## 我的代码
```
def main_operation():
print('main_operation')
all_pos = find_all(Template(r"resource/safe_state.PNG"))
print(all_pos)

def main():
t1 = Process(target=main_operation)
# t2 = Process(target=sub_operation)
t1.start()
# t2.start()
t1.join()
# t2.join()
```

### 请教下如何解决该问题，谢谢！