片段中的AsyncTask ProgressBars在onPreExecute（）方法中崩溃

Question

我有一个ASyncTask在后台线程上处理多个ProgressBars。 下面的行导致错误/崩溃。

    @Override
    protected void onPreExecute() {
        progress = maskBars[currentProgressBarId].getProgress();
    }

我很困难，因为它在演示中工作正常（在2个占位符片段内）。 我的片段是使用fragmentManager创建的，这可能是什么原因？

aSyncTask方法

    private AsyncTaskUpdateProgress asyncTaskUpdateProgress;

    public class AsyncTaskUpdateProgress extends AsyncTask<Void, Integer, Void> {
        int progress;

        @Override
        protected void onPostExecute(Void result) {
            currentProgressBarId++;

            if ((maxProgressValue - currentProgressBarId * 100 > 0) && (currentProgressBarId < maskBars.length)) {
                if (!isCancelled()) {
                    SystemClock.sleep(PROGRESS_BAR_DELAY);
                    asyncTaskUpdateProgress = new AsyncTaskUpdateProgress();
                    asyncTaskUpdateProgress.execute();
                }
            } else {
                buttonStop.setEnabled(false);
                buttonReset.setEnabled(true);
            }
        }

        @Override
        protected void onPreExecute() {
            progress = maskBars[currentProgressBarId].getProgress();
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            textViewProgress.setText(String.valueOf(crgBrId * 100 + values[0]));
            maskBars[currentProgressBarId].setProgress(values[0]);
        }

        @Override
        protected Void doInBackground(Void... args) {
            int limit = maxProgressValue - currentProgressBarId * 100;
            if (limit > 100) { limit = 100; }

            while ((!isCancelled()) && (progress < limit)) {
                progress++;

                publishProgress(progress);
                SystemClock.sleep(PROGRESS_DELAY);
            }

            return null;
        }
    }

LOG

    12-15 17:04:42.088: E/AndroidRuntime(7305): FATAL EXCEPTION: main
    12-15 17:04:42.088: E/AndroidRuntime(7305): Process: com.black.app, PID: 7305
    12-15 17:04:42.088: E/AndroidRuntime(7305): java.lang.NullPointerException
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at com.myob.app.VerticalProgressBars$AsyncTaskUpdateProgress.onPreExecute(VerticalProgressBars.java:56)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:587)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.os.AsyncTask.execute(AsyncTask.java:535)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at com.black.app.VerticalProgressBars.onClick(VerticalProgressBars.java:140)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.view.View.performClick(View.java:4445)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.view.View$PerformClick.run(View.java:18429)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.os.Handler.handleCallback(Handler.java:733)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.os.Handler.dispatchMessage(Handler.java:95)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.os.Looper.loop(Looper.java:136)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at android.app.ActivityThread.main(ActivityThread.java:5089)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at java.lang.reflect.Method.invokeNative(Native Method)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at java.lang.reflect.Method.invoke(Method.java:515)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
    12-15 17:04:42.088: E/AndroidRuntime(7305):     at dalvik.system.NativeStart.main(Native Method)
    12-15 17:04:42.095: W/ActivityManager(434):   Force finishing activity com.blagmynames.app/.NameActivity
    12-15 17:04:42.627: W/ActivityManager(434): Activity pause timeout for ActivityRecord{41b08cf8 u0 com.blackmagicnames.app/.NameActivity t29 f}
    12-15 17:04:42.767: I/ActivityManager(704): Timeline: Activity_idle id: android.os.BinderProxy@41ae0c10 time:27575356
    12-15 17:04:43.181: I/ActivityManager(434): Timeline: Activity_windows_visible id: ActivityRecord{41faaa98 u0 com.cyanogenmod.trebuchet/com.android.launcher3.Launcher t1} time:27575770