doinbackground（）中的异常

Question

我有活动A和B.他们都有AsyncTasks连接到网络并获取一些JSON。我得到一个我无法复制的错误，但我不确定我做错了什么。当我从活动B按下后退按钮到A到主屏幕时，我收到错误。

protected Void doInBackground(Void... params) {
                        synchronized (this) {
                                            if (img_flag == 1) {
                                System.out.println("entered into doinback1");
                                UploadImage();
                                System.out.println("entered into doinback2");
                            }



                            if (offlineast_flag == 0) {
                                System.out.println("entered into doinback3");
                                UpdateAsset_webService();
                                System.out.println("entered into doinback4");
                                ServiceRequest _autoupdate = new ServiceRequest();
                                _autoupdate.cat_sync(Update_asset.this, _companyId, _usrid,
                                        access_id, profile_id, catid_db);
                                System.out.println("entered into doinback5");
                            }
                            if (img_flag == 2) {
                                System.out.println("entered into doinback6");
                                UploadImageOffline();
                            } else {
                                System.out.println("entered into doinback7");
                                loading_dialog.dismiss();
                                System.out.println("entered into doinback8");
                            }
                        }
                        return null;
                    }

                    // Update the progress
                    protected void onProgressUpdate(Integer... values) {

                    }

                    // after executing the code in the thread
                    protected void onPostExecute(Void result) {
                        onback();
                        loading_dialog.dismiss();
                        img_flag = 0;
                        Toast.makeText(getApplicationContext(),
                                "Your asset updated successfully.", Toast.LENGTH_LONG)
                                .show();
                    }

这是我的Logcat：

        java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:300)
        at java.util.concurrent.Futuenter code herereTask.finishCompletion(FutureTask.java:355)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
        at java.util.concurrent.FutureTask.run(FutureTask.java:242)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:841)
        Caused by: java.lang.NullPointerException
        at com.kodology.assetbin.Update_asset.UpdateAsset_webService(Update_asset.java:1733)
        at com.kodology.assetbin.Update_asset$LoadViewTask.doInBackground(Update_asset.java:2239)
        at com.kodology.assetbin.Update_asset$LoadViewTask.doInBackground(Update_asset.java:1)
        at android.os.AsyncTask$2.call(AsyncTask.java:288)
        at java.util.concurrent.FutureTask.run(FutureTask.java:237)
        ... 4 more