Injected content script on youtube's watch page works partly

Question

I'm writing my first Firefox extension and I want to make sidebar disappear from Youtube where video recommendations are. Every time I open a video my script is injected into a page but sidebar stays unaffected until I reload the extension from about:debugging.

script works on some elements like search bar without reloading which has the id of "center" as I wrote in the code snippet below.

function hideContents() {
    //works instantly
    let center  = document.getElementById("center");
    center.style.display='none';

    //does not work until reload 
     let secondary = document.getElementById("secondary");
     if (secondary) {
        secondary.style.display = "none";
    }
  }

I tried using MutationObserver, setTimeout, Event listeners to check if page was fully loaded but nothing worked. I feel like waiting for a page to load is not a solution and I'm missing out other detail that I don't know.

What do you suggest?

Edit: I solved the issue by changing one line of code

secondary.style.display = "none"; to secondary.parentNode.removeChild(secondary);