Fix rare error with ws.close()

www/video-rtc.js

@@ -551,14 +551,18 @@ export class VideoRTC extends HTMLElement {
                 this.pcState = WebSocket.OPEN;
 
                 this.wsState = WebSocket.CLOSED;
+                if (this.ws) {
                     this.ws.close();
                     this.ws = null;
+                }
             } else {
                 this.pcState = WebSocket.CLOSED;
+                if (this.pc) {
                     this.pc.close();
                     this.pc = null;
                 }
             }
+        }
 
         video2.srcObject = null;
     }