Raspberry Pi Pico keeps crashing every since I started using both cores - multithreading

Question

I'm new to pico, only using arduinos before. I'm trying to make a simple rotary encoder program that displays a value from 0-12 on an 0.96 oled display, and lights up that many leds on a strip.
I wanted to try out using multiple cores, as interrupts made the leds not run smooth when I had them just cycling (everything would be paused while the encoder was being turned)
However, when I run this program, aside from the encoder being bouncy, the pico would crash maybe 30 seconds into running the program, making a mess on the display and stopping the code. I feel like there's some rule of using multiple cores that I completely ignored.
Here's the code:
from machine import Pin, I2C
from ssd1306 import SSD1306_I2C
import _thread
import utime
import neopixel
#general variables section
numOn = 0
#Encoder section
sw = Pin(12,Pin.IN,Pin.PULL_UP)
dt = Pin(11,Pin.IN)
clk = Pin(10,Pin.IN)
encodeCount = 0
lastClk = clk.value()
lastButton = False
#Encoder thread
def encoder(): #don't mind the indentation here,
#stackoverflow kinda messed up the code block a bit.
while True:
#import stuff that I shouldn't need to according to tutorials but it doesn't work without
global encodeCount
global lastClk
global clk
import utime
if clk.value() != lastClk:
if dt.value() != clk.value():
encodeCount += 1
else:
encodeCount -= 1
if encodeCount > 12:
encodeCount = 0
elif(encodeCount < 0):
encodeCount = 12
lastClk = clk.value()
print(encodeCount)
utime.sleep(0.01)
_thread.start_new_thread(encoder,())
#LED section
numLed = 12
ledPin = 26
led = neopixel.NeoPixel(machine.Pin(ledPin),numLed)
#Screen Section
WIDTH = 128
HEIGHT = 64
i2c = I2C(0,scl=Pin(17),sda=Pin(16),freq=200000)
oled = SSD1306_I2C(WIDTH,HEIGHT,i2c)
#loop
while True:
for i in range(numLed):
led[i] = (0,0,0)
for i in range(encodeCount):
led[i] = (100,0,0)
led.write()
#Display section
oled.fill(0)
oled.text(f'numLed: {numOn}',0,0)
oled.text(f'counter: {encodeCount}',0,40)
oled.show()
I'm probably doing something stupid here, I just don't know what.
Also, any suggestions on simply debouncing the encoder would be very helpful.
Any help would be appreciated! Thanks!
Update: The code above bricked the pico, so clearly I'm doing something very very wrong. _thread start line stopped it from crashing again, so the problem is there.

Answer 1

Same issue with very similar code on a Raspberry Pico W. I specify the 'W' because my code works without crashing on an earlier Pico.
I'm wondering if the low level networking functions might be using the 2nd core and causing a conflict.
I'm adding thread locking to see if passing a baton helps, the link below has an example, eg:
# create a lock
lck= _thread.allocate_lock()
# Function that will block the thread with a while loop
# which will simply display a message every second
def second_thread():
while True:
# We acquire the traffic light lock
lck.acquire()
print("Hello, I'm here in the second thread writting every second")
utime.sleep(1)
# We release the traffic light lock
lck.release()