Getting Cannot read properties of null (reading ‘style’)

I am trying to dynamically change the style of an element .item in my page but I am getting error

Cannot read properties of null (reading 'style')

Here is my Style

.item {
    flex: 0 0 calc(12% - 20px);
    background: #36bf00;
    color: white;
    margin: 10px 10px -20px 10px;
}

And this is my jQuery

$(document).keydown(function(e){
    var imgsize = 12;

    if (e.keyCode == 38) { // Up
        console.log("up")
        var NewImgsize = imgsize + 0.5;
        var NewStyle = '0 0 calc(' + NewImgsize + '% - 20px';
        document.getElementById('item').style.flex = NewStyle;
        return false;
    }
    if (e.keyCode == 40) { //Down
        console.log("down")
        return false;
    }
})

Any idea what I am missing?

Thanks