android - Androidアプリでヌルポインタ例外を理解できない

Question

私は次のようにコードを書きましたが、なぜコードがヌルポインタ例外を返すのか理解できませんか?

public void createXML()
{
    try
    {
        //FileOutputStream f1 = new FileOutputStream("Userdata_Boombastic.xml");
        FileOutputStream f1 = openFileOutput("Userdata_Boombastic.xml", Context.MODE_PRIVATE);
        //OutputStreamWriter out = new OutputStreamWriter(f1);
        XmlSerializer xmlSerializer = Xml.newSerializer();              
        StringWriter writer = new StringWriter();
        xmlSerializer.setOutput(writer);
        xmlSerializer.startDocument("UTF-8",true);
        xmlSerializer.endDocument();
        xmlSerializer.flush();
        String dataWrite=writer.toString();
        f1.write(dataWrite.getBytes());
        f1.close();
    }
    /*catch (FileNotFoundException e)
    {
    // TODO Auto-generated catch block
        Log.e("FileNotFoundException", e.toString());
        e.printStackTrace();
    }*/
    catch (IllegalArgumentException e)
    {
    // TODO Auto-generated catch block
        Log.e("~~IllegalArgumentException~~", e.toString());
        e.printStackTrace();
    } 
    catch (IllegalStateException e) 
    {
    // TODO Auto-generated catch block
        Log.e("~~IllegalStateException~~", e.toString());
        e.printStackTrace();
    }
    /*catch (IOException e)
    {
    // TODO Auto-generated catch block
        Log.e("IOEXCEPTION", e.toString());
        e.printStackTrace();
    }*/
    catch (Exception e) 
    {
    // TODO Auto-generated catch block
        Log.e("~~Exception~~", e.toString());
        e.printStackTrace();
    }

08-27 18:50:50.310: E/~~Exception~~(31487): java.lang.NullPointerException
08-27 18:50:57.800: E/~~Exception~~(31487): java.lang.NullPointerException
08-27 18:51:00.430: E/~~Exception~~(31487): java.lang.NullPointerException
08-27 18:53:28.050: E/ExternalAccountType(30234): Unsupported attribute readOnly
08-27 18:53:29.680: E/ExternalAccountType(30234): Unsupported attribute readOnly
08-27 18:53:32.500: E/~~Exception~~(32054): java.lang.NullPointerException
08-27 18:53:51.670: E/~~Exception~~(32054): java.lang.NullPointerException

他の説明のために、私はこのコードをPCに接続してセルで実行しています。

助けてください

まあ、スタックトレースは実りあるものを何も返さなかったと思います

08-27 19:30:31.330: E/ExternalAccountType(30234): Unsupported attribute readOnly
08-27 19:30:31.820: E/ExternalAccountType(30234): Unsupported attribute readOnly
08-27 19:30:36.030: E/~~Exception~~(2732): java.lang.NullPointerException
08-27 19:30:36.150: E/->>(2732): ~~stacktrace~~
08-27 19:30:36.150: E/->>(2732): java.lang.NullPointerException
08-27 19:30:36.150: E/->>(2732):    at           android.content.ContextWrapper.openFileOutput(ContextWrapper.java:165)
08-27 19:30:36.150: E/->>(2732):    at com.example.boombastic.WritingXML.createXML(WritingXML.java:76)
08-27 19:30:36.150: E/->>(2732):    at com.example.boombastic.BoombasticPlayer.onCreate(BoombasticPlayer.java:22)
08-27 19:30:36.150: E/->>(2732):    at android.app.Activity.performCreate(Activity.java:4470)
08-27 19:30:36.150: E/->>(2732):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
08-27 19:30:36.150: E/->>(2732):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
08-27 19:30:36.150: E/->>(2732):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
08-27 19:30:36.150: E/->>(2732):    at android.app.ActivityThread.access$600(ActivityThread.java:123)
08-27 19:30:36.150: E/->>(2732):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
08-27 19:30:36.150: E/->>(2732):    at android.os.Handler.dispatchMessage(Handler.java:99)
08-27 19:30:36.150: E/->>(2732):    at android.os.Looper.loop(Looper.java:137)
08-27 19:30:36.150: E/->>(2732):    at android.app.ActivityThread.main(ActivityThread.java:4424)
08-27 19:30:36.150: E/->>(2732):    at java.lang.reflect.Method.invokeNative(Native Method)
08-27 19:30:36.150: E/->>(2732):    at java.lang.reflect.Method.invoke(Method.java:511)
08-27 19:30:36.150: E/->>(2732):    at    com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:817)
08-27 19:30:36.150: E/->>(2732):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:584)
08-27 19:30:36.150: E/->>(2732):    at dalvik.system.NativeStart.main(Native Method)