在用官网的yolov5模型训练时,出现这个错误,错误代码如下:
```python
Traceback (most recent call last):
File "e:\yolov5\yolov5-master\train.py", line 643, in
main(opt)
File "e:\yolov5\yolov5-master\train.py", line 532, in main
train(opt.hyp, opt, device, callbacks)
File "e:\yolov5\yolov5-master\train.py", line 113, in train
data_dict = data_dict or check_dataset(data) # check if None
File "e:\yolov5\yolov5-master\utils\general.py", line 489, in check_dataset
assert k in data, emojis(f"data.yaml '{k}:' field missing ❌")
TypeError: argument of type 'NoneType' is not iterable
```请问如何解决