fix: Fix TypeError when reloading the web page of the Receiver sample (#786)

Author: karasusan

WebApp/client/public/receiver/js/main.js

@@ -32,6 +32,8 @@ window.addEventListener('resize', function () {
 }, true);
 
 window.addEventListener('beforeunload', async () => {
+  if(!renderstreaming)
+    return;
   await renderstreaming.stop();
 }, true);