＜OpenCv-Python＞cv.convexityDefects函数返回defects时，调用defects.shape时报错的处理

OpenCv有一个凸包缺陷检测的函数cv.convexityDefects,其官方示例如下：

import cv2 as cv
import numpy as np
img = cv.imread('star.jpg')
img_gray = cv.cvtColor(img,cv.COLOR_BGR2GRAY)
ret,thresh = cv.<