How do I understand why the reading classlist part is null?

I’m trying to make a button in a website to hide/show my slides, and replace it with a different component. I did it before and it worked out, so I thought a similar code would work but it doesn’t.

Here is the html of my button:

<li><div class="behandlung-btn">Meine Behandlung</div></li>

And here is the html of the start of sliders:

<div class="sliders hideContent">
  <div class="slider home-slider clearfix" data-auto-play="8000">

Of course I closed the tag but since it’s pretty long I didn’t want to paste it all.

And here is the vanilla JS I tried:

   let showMoreBtn = document.querySelectorAll(".behandlung-btn");
  showMoreBtn.forEach((eachBtn) => {
    let textContainer = eachBtn.parentNode;
    let contentBox = textContainer.querySelector(".sliders").classList;
    eachBtn.addEventListener("click", (event) => {
      event.preventDefault();
      if (contentBox.contains("showContent")) {
        contentBox.remove("showContent");
        contentBox.add("hideContent");
        eachBtn.innerHTML = `<h12>ZEIG WENIGER</h12>`;
      } else {
        contentBox.add("showContent");
        contentBox.remove("hideContent");
        eachBtn.innerHTML = `<h12>ZEIG MEHR</h12>`;
      }
    });
  });

But it still says

Uncaught TypeError: Cannot read properties of null (reading 'classList')
at index.html:436:65
at NodeList.forEach (<anonymous>)
at index.html:434:19

I know there are a lot of questions in Stackoverflow about this typeError and I checked them out, but still couldn’t understand where the problem is, and also I’m a bit overwhelmed. Please help me out.

Thanks for the help in advance people!

>Solution :

It seems there is no element with .sliders class inside the textContainer element which is the parent of the button whose class behandlung-btn

You need to use document instead.

let contentBox = document.querySelector(".sliders").classList;