diff --git a/src/background/actions.ts b/src/background/actions.ts
index 5ae145b..70d5226 100644
--- a/src/background/actions.ts
+++ b/src/background/actions.ts
@@ -1,6 +1,6 @@
 import { Actions, Request } from "../common";
 import { sendMessage, ResponseChecker } from "./messaging";
-import { logger } from "./logger";
+import { logger } from "../common/logger";
 
 /**
  * redirect tab to url.
diff --git a/src/background/caches.ts b/src/background/caches.ts
index c51f2ea..e085d6e 100644
--- a/src/background/caches.ts
+++ b/src/background/caches.ts
@@ -1,4 +1,4 @@
-import { logger } from "./logger";
+import { logger } from "../common/logger";
 import { Actions } from "../common";
 import { messageSubscribers } from "./messaging";
 
diff --git a/src/background/extractor.ts b/src/background/extractor.ts
index 56a5a81..6eaac2d 100644
--- a/src/background/extractor.ts
+++ b/src/background/extractor.ts
@@ -2,7 +2,7 @@ import { Task } from "./task";
 import { saveFile } from "./tools";
 import { createTab, getActiveTab, ping } from "./actions";
 import { ExtractResult } from "./result";
-import { logger } from "./logger";
+import { logger } from "../common/logger";
 import { caches } from "./caches";
 
 export class Extractor {
@@ -12,6 +12,14 @@ export class Extractor {
     constructor(options?) {
         if (options) this._options = options;
     }
+    static async ping(count: number = 1) {
+        let tab = await getActiveTab(true) || await getActiveTab(false);
+        let succ = await ping(tab, count);
+        if (!succ) {
+            logger.error('Cannot contact with active tab.');
+            return;
+        }
+    }
     /**
      * Save current state, in case we restore it later.
      */
diff --git a/src/background/messaging.ts b/src/background/messaging.ts
index d5c3357..06b7eaa 100644
--- a/src/background/messaging.ts
+++ b/src/background/messaging.ts
@@ -1,6 +1,6 @@
 import { Request, Actions, Response } from "../common";
 import { getTabByID } from "./actions";
-import { logger } from "./logger";
+import { logger } from "../common/logger";
 
 
 export type ResponseCheckerSync<T> = (r: Response<T>, err: chrome.runtime.LastError, count: number) => T;
diff --git a/src/background/result.ts b/src/background/result.ts
index b3e795a..75cd840 100644
--- a/src/background/result.ts
+++ b/src/background/result.ts
@@ -26,6 +26,7 @@ export class ExtractResult {
                 }
                 let line = lineCells.reduce(
                     (lineText, cell, idx) => {
+                        cell = cell || "";
                         cell = '"' + cell.trim().replace(/"/g, '""') + '"';
                         return lineText + cell + (idx == lineCells.length - 1 ? "" : ",")
                     }, "");
diff --git a/src/background/task.ts b/src/background/task.ts
index 93230c5..be49c36 100644
--- a/src/background/task.ts
+++ b/src/background/task.ts
@@ -4,7 +4,7 @@ import { testArgs, signitures } from "./signiture";
 import { ExtractResult } from "./result";
 import { messageSubscribers, ActionSubscriber } from "./messaging";
 import { Actions } from "../common";
-import { logger } from "./logger";
+import { logger } from "../common/logger";
 
 export class Task {
     private _data: { [key: string]: string[][] } = {};
@@ -87,7 +87,7 @@ export class Task {
             logger.info(`Watcher #${taskID} starts.`);
             let pm = this.makeOptionalTasks(sender.tab);
             return pm.then(
-                () => extractTabData(sender.tab, this._itemsSelector, this._fieldSelectors, sender.tab.url, false)
+                () => extractTabData(sender.tab, this._itemsSelector, this._fieldSelectors, sender.tab.url, true)
             ).then(
                 results => {
                     if (results && results.length) {
diff --git a/src/common.ts b/src/common/index.ts
similarity index 100%
rename from src/common.ts
rename to src/common/index.ts
diff --git a/src/background/logger.ts b/src/common/logger.ts
similarity index 94%
rename from src/background/logger.ts
rename to src/common/logger.ts
index 4f2ff68..2c32c38 100644
--- a/src/background/logger.ts
+++ b/src/common/logger.ts
@@ -13,7 +13,7 @@ export class Logger {
     constructor(logLevel, notifyLevel) {
         if (logLevel) this._log_level = logLevel;
         if (notifyLevel) this._notify_level = notifyLevel;
-        chrome.notifications.onClosed.addListener((id, byUser) => { this._notify_level = undefined });
+        if (chrome.notifications) chrome.notifications.onClosed.addListener((id, byUser) => { this._notify_level = undefined });
     }
     get logLevel() {
         return this._log_level;