availablity check before run on active tab

scripts/background/extractor.js

@@ -56,6 +56,11 @@ class Extractor {
             tab = await createTab(task.urls[0], false);
         } else {
             tab = await getActiveTab(true) || await getActiveTab(false);
+            let succ = await ping(tab);
+            if (!succ) {
+                console.log('Cannot contact with active tab.');
+                return;
+            }
         }
         this._running = true;
         return this._tasks.reduce((pms, task, i) => {