TypeError: 'NoneType' object is not subscriptable while I'm running lane detection code

Question

I'm very new to python. I'm using google colab because I'm using laptop. I imported https://github.com/Dt-Pham/Advanced-Lane-Lines.git and followed instructions the youtube. Here's the site, https://www.youtube.com/watch?v=iRTuCYx6quQ&ab_channel=MisbahMohammed. The code worked fine for existing video clip. But when I used my own video clip to check the lane detection, it showed this error

<decorator-gen-171> in set_make_frame(self, mf)

<decorator-gen-124> in get_frame(self, t)

/content/Advanced-Lane-Lines/LaneLines.py in fit_poly(self, img)
    176         ploty = np.linspace(miny, maxy, img.shape[0])
    177 
--> 178         left_fitx = self.left_fit[0]*ploty**2 + self.left_fit[1]*ploty + self.left_fit[2]
    179         right_fitx = self.right_fit[0]*ploty**2 + self.right_fit[1]*ploty + self.right_fit[2]
    180 

TypeError: 'NoneType' object is not subscriptable

I'm not sure why this happened... the video width and height are both same(1280*720). For more detatils, here's my code

import numpy as np
import matplotlib.image as mpimg
import cv2
from docopt import docopt
from IPython.display import HTML
from IPython.core.display import Video
from moviepy.editor import VideoFileClip
from CameraCalibration import CameraCalibration
from Thresholding import *
from PerspectiveTransformation import *
from LaneLines import *

class FindLaneLines:
    def __init__(self):
        """ Init Application"""
        self.calibration = CameraCalibration('camera_cal', 9, 6)
        self.thresholding = Thresholding()
        self.transform = PerspectiveTransformation()
        self.lanelines = LaneLines()

    def forward(self, img):
        out_img = np.copy(img)
        img = self.calibration.undistort(img)
        img = self.transform.forward(img)
        img = self.thresholding.forward(img)
        img = self.lanelines.forward(img)
        img = self.transform.backward(img)

        out_img = cv2.addWeighted(out_img, 1, img, 0.6, 0)
        out_img = self.lanelines.plot(out_img)
        return out_img

    def process_image(self, input_path, output_path):
        img = mpimg.imread(input_path)
        out_img = self.forward(img)
        mpimg.imsave(output_path, out_img)

    def process_video(self, input_path, output_path):
        clip = VideoFileClip(input_path)
        out_clip = clip.fl_image(self.forward)
        out_clip.write_videofile(output_path, audio=False)

def main():
    findLaneLines = FindLaneLines()
    findLaneLines.process_video("test_sample.mp4","output.mp4")

if __name__ == "__main__":
    main()

google colab said

img = self.lanelines.forward(img)

there's the problem(red line).

Very thank you!!