ref(external-api) set ifrma source last

Make sure the ifrme is fully configured before setting the source URL
and thus triggering the load.

modules/API/external/external_api.js

@@ -388,7 +388,6 @@ export default class JitsiMeetExternalAPI extends EventEmitter {
 
         this._frame = document.createElement('iframe');
         this._frame.allow = 'camera; microphone; display-capture; autoplay; clipboard-write';
-        this._frame.src = this._url;
         this._frame.name = frameName;
         this._frame.id = frameName;
         this._setSize(height, width);
@@ -403,6 +402,8 @@ export default class JitsiMeetExternalAPI extends EventEmitter {
         }
 
         this._frame = this._parentNode.appendChild(this._frame);
+
+        this._frame.src = this._url;
     }
 
     /**