(VanillaJS) Problem with div following cursor with mouseover

I am trying to make a div follow my cursor while the cursor is inside another div and I am getting this error.

mouseOver.js:23 Uncaught TypeError: Cannot read property ‘style’ of null
at HTMLDivElement. (mouseOver.js:23)
That’s whats on line 23: m.style.top = e.clientY + ‘px’;

The code works partially. The div that is supposed to follow the cursor just jumps on the X axis, but not on the Y. And the error appears occasionally, not all the time.

const circle = document.querySelector('.item');
// const bdy = document.addEventListener('mousemove',function(e){
//     mX = e.clientX;
//     mY = e.clientY;
//     console.log(mX);
// });

// let mX;
// let mY;

    this.appendChild(popUpInformation('Show some information about this blyat.Show some information about this blyat.Show some information about this blyat.Show some information about this blyat.'));


    //this.childNodes[1].style.top = e.clientY +'px';
    //this.childNodes[1].style.left = e.clientX - 115 +'px';
    let m = document.querySelector('.moving');
    m.style.top = e.clientY + 'px';
    m.style.left = e.clientX + 'px';


function popUpInformation(infoText,e){
    let newElem = document.createElement('div');
    newElem.innerHTML = infoText;

    newElem.className = 'moving';
    newElem.style.position = 'absolute';
    newElem.style.backgroundColor = '#FDDFDF';
    newElem.style.borderRadius = '5px';
    newElem.style.padding = '15px';
    newElem.style.maxWidth = '230px';
    newElem.style.top = 'inherit';
    newElem.style.left = 'inherit';
    return newElem;

Do you have a sample visual?