Javascript NullPointerException. The same below code is running errors free in my Ide but get errors in sololearn ide

Question

const burElement = document.querySelector(".humburger")

const navElement = document.querySelector(".nav");

burElement.addEventListener("click",()=>{
navElement.classList.toggle("nav-open");

burElement.classList.toggle("hum-open")
});

navElement.addEventListener("click",()=>{
navElement.classList.remove("nav-open");

burElement.classList.remove("hum-open")
});

https://sololearn.com/compiler-playground/WaHPSa9E6UiN/?ref=app