Add checks for container presence in animation functions to prevent errors when elements are removed from the DOM [skip ci]

Jellyfin.Plugin.Seasonals/Web/matrix.js

@@ -136,6 +136,7 @@ function createElements() {
   for(let i=0; i<maxTrails; i++) trails.push(new Trail());
 
   function loop() {
+      if (!document.body.contains(container)) { clearInterval(window.matrixInterval); return; }
       if (isHidden) return; // Pause drawing when hidden
       ctx.clearRect(0, 0, canvas.width, canvas.height);
       ctx.font = 'bold ' + fontSize + 'px monospace';