Update iframe.js (#2443)

* Update iframe.js

Wappalyzer not work with Firebase
Chrome 68
MacOs 10.13
Uncaught TypeError: Cannot read property 'postMessageId' of null at onPostMessage (iframe.js:1000)

* Update iframe.js

* Update iframe.js

src/drivers/webextension/js/lib/iframe.js

@@ -997,6 +997,8 @@
         return;
       }
 
+      if (!adData) return;
+      
       if (adData.postMessageId === POST_MSG_ID) {
         delete adData.postMessageId;