Menu mobile doesnt'work any more with css : active selector

Hello, I’ve a navigation menu which is hidden when the breakpoint is 640px . When I added an active element to it, my menu navigation works but my mobile menu stops working and I’ve got a message ‘Cannot read property ‘classList’ of null at HTMLBodyElement’.
Could you help me resolve this problem without deleting my active element?
Thanks for your help!

.menuMobile {

      position: absolute;

      margin-top: 10%;

      margin-left: 10%;

      margin-right: 10%;

      margin-bottom: 35%;

      height: 62%;

      @media only screen and (min-width: $sm) {

        visibility: visible !important;

        background-color: white;

      }

    }

    @media only screen and (min-width: $sm) {

  .active{

      text-decoration: underline;

    }}

    .mobile {

      position: absolute;

      width: 100%;

      height: 140%;

      z-index: 900;

      padding: 40% 10% 10% 20%;

      margin-left: 0%;

      margin-right: 0%;

      background-color: black;

      // left : -100%;

      animation: run 1s cubic-bezier(0.3, 0.460, 0.450, 0.940);

      animation-duration: 1s;

    }

    @keyframes run {

      0% {

        right: 100%;

      }

      100% {

        right: 0;     

      }

    }
<div class="sous-menu-burger">

        <i class="fas fa-grip-lines"></i>

    </div> 

    <div class="cont-main">

        <div id="menuMobile" class="menuMobile"> 

                <a href="#"><div class="home-a">Home</div></a>

                <a class="active" href="#"><div class="about-a">About</div></a>

                <a href="#"><div class="skills-a">Skills</div></a>

                <a href="#"><div class="projects-a">Projects</div></a>

                <a href="#"><div class="contacts-a">Contacts</div></a>

        </div>
let elem = document.querySelector("#menuMobile");

document.body.addEventListener("click", function (e) {

  if (e.target.matches(".fa-grip-lines")) {

    

    if (menuMobile.style.visibility === "hidden" && window.innerWidth < "640") {

      elem.classList.remove("menuMobile");

      elem.classList.add("mobile");

      elem.style.visibility = "visible";

      e.target.style.color = "white";

    } else if (

      menuMobile.style.visibility != "visible" &&

      window.innerWidth > "640"

    ) {

      elem.classList.remove("mobile");

      

    } 

    else {

      menuMobile.style.visibility = "hidden";

      elem.classList.remove("mobile");

      elem.classList.add("menuMobile");

      e.target.style.color = "black";

      

    }

  }

});

Hi @toffilek,

Verify the syntax for document.querySelector statement to select menuMobile class in your JavaScript.

Thanks, I corrected my code and it works.