Error: (-215:Assertion failed) !_img.empty() in function 'imwrite' why does it say the file is empty?

Question

Python

I write in pycharm. On line 30, there is an error Error: (-215:Assertion failed) !_img.empty() in function 'imwrite'
All files exist, they are in the library with the code.

import numpy as np
import cv2
import copy
from make_video import make_video
from progress.bar import Bar


def main():
    capture = cv2.VideoCapture('./avaria.mp4')
    background_subtractor = cv2.bgsegm.createBackgroundSubtractorMOG()
    length = int(capture.get(cv2.CAP_PROP_FRAME_COUNT))

    bar = Bar('Processing Frames', max=length)

    first_iteration_indicator = 1
    for i in range(0, length):

        ret, frame = capture.read()

        # If first frame
        if first_iteration_indicator == 1:

            first_frame = copy.deepcopy(frame)
            height, width = frame.shape[:2]
            accum_image = np.zeros((height, width), np.uint8)
            first_iteration_indicator = 0
        else:

            filter = background_subtractor.apply(frame)  # remove the background
            cv2.imwrite('./frame.jpg', frame) # ТУТ///////////////
            cv2.imwrite('./diff-bkgnd-frame.jpg', filter)

            threshold = 2
            maxValue = 2
            ret, th1 = cv2.threshold(filter, threshold, maxValue, cv2.THRESH_BINARY)

            # add to the accumulated image
            accum_image = cv2.add(accum_image, th1)
            cv2.imwrite('./mask.jpg', accum_image)

            color_image_video = cv2.applyColorMap(accum_image, cv2.COLORMAP_SUMMER)

            video_frame = cv2.addWeighted(frame, 0.7, color_image_video, 0.7, 0)

            name = "./frames/frame%d.jpg" % i
            cv2.imwrite(name, video_frame)

            if cv2.waitKey(1) & 0xFF == ord('q'):
                break
        bar.next()

    bar.finish()

    make_video('./frames/', './output.avi')

    color_image = cv2.applyColorMap(accum_image, cv2.COLORMAP_HOT)
    result_overlay = cv2.addWeighted(first_frame, 0.7, color_image, 0.7, 0)

    # save the final heatmap
    cv2.imwrite('diff-overlay.jpg', result_overlay)

    # cleanup
    capture.release()
    cv2.destroyAllWindows()


if __name__ == '__main__':
    main()

I took the code from the guide: https://github.com/robertosannazzaro/motion-heatma...