Need Help About JavaScript Error Log

Dear All

I have this code ->

if("serviceWorker" in navigator){
navigator.serviceWorker.register("/service.js");}

if(localStorage.getItem("preferredTheme")==="dark"){setDarkMode(true);}
function setDarkMode(isDark){var darkBtn=document.getElementById("darkBtn");
var lightBtn=document.getElementById("lightBtn");
if(isDark){lightBtn.style.display="block";
darkBtn.style.display="none";
localStorage.setItem("preferredTheme","dark");}else{lightBtn.style.display="none";
darkBtn.style.display="block";
localStorage.removeItem("preferredTheme");}
document.body.classList.toggle("darkmode");}
document.getElementById("darkBtn").addEventListener("click", function(){setDarkMode(true);});
document.getElementById("lightBtn").addEventListener("click", function(){setDarkMode(false);});

And I got this warning from LightHouse ->

TypeError: Cannot read property 'addEventListener' of null at https://angora.me/script.js :13:35

Any suggestion how to fix it?

Regards