title: OpenCV version 4.1.0 drawContours error: (-215:Assertion failed) npoints > 0 in function 'drawContours'
body: <p>I have the following code that worked well with OpenCV 3.4.1 and now is not working with OpenCV 4.1.0 and gives an error. I do not know how to adapt the code with the newer version, can you help me with that? Thanks a lot</p>
  
  <pre><code>def ImageProcessing(image):
      image = cv2.absdiff(image, background)
      h, gray = cv2.threshold(image, 65, 255, cv2.THRESH_BINARY_INV);
      gray = cv2.medianBlur(gray,5)
  
      kernel = np.ones((3,3), np.uint8)
  
      gray = cv2.erode(gray, kernel, iterations=1)#1
  
      des = cv2.bitwise_not(gray)
      tmp = cv2.findContours(des,cv2.RETR_CCOMP,cv2.CHAIN_APPROX_SIMPLE)
      contour, hier = tmp[1], tmp[0]
  
      for cnt in contour:
          cv2.drawContours(des,[cnt],0,255,-1)
  
      gray = cv2.bitwise_not(des)
  
      gray = cv2.dilate(gray, kernel, iterations=1)#1
  
      return gray
  </code></pre>
  
  <p>The error is </p>
  
  <blockquote>
    <p>cv2.error: OpenCV(4.1.0) /io/opencv/modules/imgproc/src/drawing.cpp:2509: error: (-215:Assertion failed) npoints > 0 in function 'drawContours'</p>
  </blockquote>