addEventListener -> font awesome

Question

:) So I cant figure it out how to addeventlistener to an icon. I have something like that in HTML

   <li><button><i class="fas fa-moon"></i></button></li>

aaaand something like that in JS

   var nightMode = document.querySelector("button");

nightMode.addEventListener("click", function(){
console.log("it's working");

});

but it's not working. Uncaught TypeError: Cannot read property 'addEventListener' of null Can someone explain me what to do and why ? I also add in css cursor: pointer; for that button :|