从onPostExecute更新UI时间歇性崩溃

Question

这是stacktrace：

06-10 09:14:18.149: E/AndroidRuntime(1230): FATAL EXCEPTION: main
06-10 09:14:18.149: E/AndroidRuntime(1230): java.lang.NullPointerException
06-10 09:14:18.149: E/AndroidRuntime(1230):   at ca.TwentyTwenty.cropinspection.GeneralSpinnerDialogFragment$AsyncSaver.onPostExecute(GeneralSpinnerDialogFragment.java:140)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at ca.TwentyTwenty.cropinspection.GeneralSpinnerDialogFragment$AsyncSaver.onPostExecute(GeneralSpinnerDialogFragment.java:1)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at android.os.AsyncTask.finish(AsyncTask.java:631)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at android.os.AsyncTask.access$600(AsyncTask.java:177)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at android.os.Handler.dispatchMessage(Handler.java:99)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at android.os.Looper.loop(Looper.java:137)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at android.app.ActivityThread.main(ActivityThread.java:5041)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at java.lang.reflect.Method.invokeNative(Native Method)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at java.lang.reflect.Method.invoke(Method.java:511)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
06-10 09:14:18.149: E/AndroidRuntime(1230):   at dalvik.system.NativeStart.main(Native Method)

这是代码

FieldDetailItem fd = ((FieldActivity) getActivity()).fieldDetails.get(list_position);

我很确定这与asDctask回来时可能不存在的fieldDetails有关，但我不知道还有什么可以做的。我有什么选择？

感谢。