title: cv2.error (-215:Assertion failed) reader.ptr != NULL in function cvDrawContours
body: <p>So I'm trying to create a license plate detection program and I have been following a guide for that,
  (<a href="https://github.com/nicknochnack/ANPRwithPython/blob/main/ANPR%20-%20Tutorial.ipynb" rel="nofollow noreferrer">https://github.com/nicknochnack/ANPRwithPython/blob/main/ANPR%20-%20Tutorial.ipynb</a>)
  however I am currently running into a problem.</p>
  <pre><code>img = cv2.imread('image4.jpg')
  gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  
  bfilter = cv2.bilateralFilter(gray, 11, 17, 17) #Noise reduction
  edged = cv2.Canny(bfilter, 30, 200) #Edge detection
  
  keypoints = cv2.findContours(edged.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
  contours = imutils.grab_contours(keypoints)
  contours = sorted(contours, key=cv2.contourArea, reverse=True)[:10]
  
  location = None
  for contour in contours:
      approx = cv2.approxPolyDP(contour, 10, True)
      if len(approx) == 4:
          location = approx
          break
  
  mask = np.zeros(gray.shape, np.uint8)
  new_image = cv2.drawContours(mask, [location], 0,255, -1)
  new_image = cv2.bitwise_and(img, img, mask=mask)
  </code></pre>
  <p>I have also tried to change &quot;location&quot; to 0 or [0] but to no success.</p>