I have a doubt about the JavaScript element assignment. Here is my code for the "save" function. May I know why the code below shows error:

Cannot read properties of null (reading 'textContent')

let saveEl = document.getElementById("save-el");
function save(){
    let countStr = count + " - ";
    saveEl.textContent += countStr //This line doesn't worked.
    console.log(count);
}

But, when I removed the variable assignment saveEl as shown below, the code works well.

function save(){
    let countStr = count + " - ";
    document.getElementById("save-el").textContent += countStr;
    console.log(count);
}

Can anyone here explain to me why my code behave this way?