UnboundLocalError in Obstacle Detection with Ultrasonic Sensor Using MicroPython

from machine import Pin
import time

# Ultrasonic sensor setup
trigger = Pin(5, Pin.OUT)
echo = Pin(18, Pin.IN)

def measure_distance():
    trigger.off()
    time.sleep_us(2)
    trigger.on()
    time.sleep_us(10)
    trigger.off()

    while echo.value() == 0:
        start = time.ticks_us()
    while echo.value() == 1:
        end = time.ticks_us()

    duration = time.ticks_diff(end, start)
    distance = (duration * 0.0343) / 2
    return distance

while True:
    distance = measure_distance()
    print('Distance:', distance, 'cm')
    if distance < 10:
        print("Obstacle detected! Stopping car.")
        stop()  # Add stop function from Stage 1
    time.sleep(1)