Safe guard for removed parent node of the iframe.

modules/API/external/external_api.js

         this.emit('_willDispose');
         this._transport.dispose();
         this.removeAllListeners();
-        if (this._frame) {
+        if (this._frame && this._frame.parentNode) {
             this._frame.parentNode.removeChild(this._frame);
         }
     }