About the exercise in 'OpenCV-Python Tutorials - Trackbar as the Color Palette'

Update:

It works fine now.

Original:

question from the exercise in the link: http://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_gui/py_trackbar/py_trackbar.html#trackbar

I want to create a Paint application with adjustable colors and brush radius using trackbars. But when I run my code for some seconds, my python.exe shut down, it seems that there is a memory access violation but I don't know why.

Thanks for your help!

import cv2
import numpy as np

draw = False

def nothing(x):
    pass
# use mouse to draw lines
def draw_line(event,x,y,flags,param):
    global draw
    if event == cv2.EVENT_LBUTTONDOWN:
        draw = True
    elif event == cv2.EVENT_MOUSEMOVE:
        if draw == True:
            cv2.circle(img, (x,y),t,(b,g,r),-1)
    elif event == cv2.EVENT_LBUTTONUP:
        draw = False
    else:
        pass
# get current positions of five trackbars
def get_tar():
    global r,g,b,s,t
    r = cv2.getTrackbarPos('R','image')
    g = cv2.getTrackbarPos('G','image')
    b = cv2.getTrackbarPos('B','image')
    s = cv2.getTrackbarPos(switch,'image')
    t = cv2.getTrackbarPos('T','image')

# Create a black image, a window
img = np.zeros((300,512,3),np.uint8)
img[:] = 255
cv2.namedWindow('image')

# create trackbars for color and thickness change
cv2.createTrackbar('R','image',0,255,nothing)
cv2.createTrackbar('G','image',0,255,nothing)
cv2.createTrackbar('B','image',0,255,nothing)
cv2.createTrackbar('T','image',0,20,nothing)

# create switch for ON/OFF and reset functionality
switch = '0 : OFF \n1 : ON'
cv2.createTrackbar(switch, 'image',0,1,nothing)

while(1):
    cv2.imshow('image',img)
    k = cv2.waitKey(100) & 0xFF
    if k == 27:
        break

 # get current positions of five trackbars
    get_tar()

    if s == 0:
        img[:] = 255
        draw = False
    else:
        cv2.setMouseCallback('image', draw_line)

cv2.destroyAllWindows()

and the error is:

Process finished with exit code -1073741819 (0xC0000005)

and then my python.exe shut down

I should put the

cv2.setMouseCallback('image', draw_line) 

out of the 'while' loop, or there will be a memory error