do not listen on camera control button when it's not present

static/sfu.js

@@ -1092,6 +1092,8 @@ function registerControlEvent(peerid) {
       }
   };
 
+  let camera = peer.querySelector("span.camera");
+  if (camera) {
       peer.querySelector("span.camera").onclick = function(event) {
           event.preventDefault();
           let video = getParentVideo(event.target);
@@ -1108,6 +1110,7 @@ function registerControlEvent(peerid) {
               addLocalMedia(id);
           }
       };
+  }
 }