cv2 Stitcher and Thread Behaving Strange

Question

I am trying to stitch 16 images using Stitcher from cv2 on RPi 4 8GB RAM, when I use it along with Thread it is not working Below is python script :

import cv2
import numpy as np
import os
import time
from threading import Thread
cv2.ocl.setUseOpenCL(True)

global images
images = []
imgreg_path = "/home/pi/shared/"

class t1:
    def __init__(self):
        self._running = True

    def terminate(self):  
        self._running = False
    def run(self):
        global images
        global stitcher
        stitcher = cv2.Stitcher_create(mode=1)
        print("Thread 1 running")
        print(len(images))
        (status, stitched) = stitcher.stitch(images)
        if status == 0:
            cv2.imwrite('/home/pi/Desktop/stitched.jpg', stitched)
            print('Thread 1 Stitched')
            images = []
        else:
            print('Thread 1 Not Stitched')
            images = []

for i in range(2,18):
    curImg = cv2.imread(imgreg_path+"image"+str(i)+".jpg")
    images.append(curImg)
print(len(images))

task1 = t1()
thrd1 = Thread(target=task1.run)
thrd1.start()
while True:
    time.sleep(5)

and OUTPUT

>>> %Run testsew.py
16
Thread 1 running
16
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/home/pi/Desktop/testsew.py", line 24, in run
    (status, stitched) = stitcher.stitch(images)
cv2.error: OpenCV(4.1.0) /home/pi/opencv-python/opencv/modules/core/src/umatrix.cpp:454: error: (-215:Assertion failed) u != 0 in function 'create'

However it works perfectly without threading, below is python script :

import cv2
import numpy as np
import os
import time
from threading import Thread
cv2.ocl.setUseOpenCL(True)

imgreg_path = "/home/pi/shared/"
stitcher = cv2.Stitcher_create(mode=1)
images = []
print("Running")
for i in range(2,18):
    curImg = cv2.imread(imgreg_path+"image"+str(i)+".jpg")
    images.append(curImg)
print(len(images))
(status, stitched) = stitcher.stitch(images)
if status == 0:
    cv2.imwrite('/home/pi/Desktop/stitched.jpg', stitched)
    print('Stitched')
    images = []
else:
    print('Not Stitched')
    images = []

and OUTPUT

>>> %Run testsew_read.py
Running
16
Stitched

Any help is appreciated to achieve stitching along with threading. Thank you.