wait for elements

scripts/background/actions.js

@@ -105,7 +105,7 @@ function extractTabData(tab, itemsSelector, fieldSelectors) {
         itemsSelector: itemsSelector,
         fieldSelectors: fieldSelectors
     }
-    let cond = r => r !== undefined;
+    let cond = r => !MSG_ELEMENT_NOT_FOUND.isEqual(r);
     return sendMessage(tab, req, 'Extract data from the tab...', cond);
 }