First remove the image = cv2.imread(img_path) line. You read the image only once and then it becomes an object and you can't use cv2.imread() on an object. After doing this there error is still there then it means that your image is not in the same folder where the kernel is running from. You can check this using

import os
os.path.exists('6.jpg') # Will likely give False
os.getcwd() # This will return the current working directory

After this check, you will likely get false. Now you should look at the relative path of your image from your current working directory and pass that path into your cv2.imread().

From Python: Attribute Error - 'NoneType' object has no attribute 'something':

NoneType means that instead of an instance of whatever Class or Object you think you're working with, you've actually got None. That usually means that an assignment or function call up above failed or returned an unexpected result.

This means that there are probably some features (geometries) that are not valid. You may check your input layer using the Check validity algorithm available from the Processing Toolbox and then try again to run the Polygonize tool. It will allow you to repair any eventually invalid geometry.

If it still doesn't work, you may also try the Lines to polygon tool or the Convert lines to polygons SAGA tool, which are both available from the Processing Toolbox.

Answering, because the community brought it back. Adding my two objects (cents).

The only reason you see that error is because you are trying to get information or perform operations on an object that doesn't exist in the first place. To check, try printing the object. Like, adding -

print img      # such as this case
print contours # if you are working with contours and cant draw one
print frame    # if you are working with videos and it doesn't show

gives you a None. That means you haven't read it properly. Either the image name you gave does not exist or the path to it is wrong. If you find such an error here's the quick things to do-

1. Check the path or bring your image to the working directory
2. Check the name you gave is right (including the extension- .jpg, .png etc)
3. Put the entire code in a if statement with the object and the code to proceed if true
4. Will add more if suggested in the comments.