why i get java.lang.NullPointerException when I using jexcel api?
<p>i using jexcel api to write excel document</p>

<pre><code>package com.xls;

import android.app.Activity;
import android.os.Bundle;
import java.io.File;
import java.io.IOException;
import jxl.*;
import jxl.write.*; 
import jxl.write.Number;
import jxl.write.biff.RowsExceededException;

public class ExcelActivity extends Activity {
    WritableWorkbook workbook;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        try {
            workbook = Workbook.createWorkbook(new File("output.xls"));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 
        WritableSheet sheet = workbook.createSheet("First Sheet", 0); 

        Label label = new Label(0, 2, "A label record");
        try {
            sheet.addCell(label);
        } catch (RowsExceededException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        } catch (WriteException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }

        Number number = new Number(3, 4, 3.1459);
        try {
            sheet.addCell(number);
        } catch (RowsExceededException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (WriteException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }


        try {
            workbook.write();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        try {
            workbook.close();
        } catch (WriteException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 
    }
}
</code></pre>

<p>why i get force close ?? i've adding permission write external storage at manifest.</p>

<pre>
05-04 11:14:45.879: ERROR/AndroidRuntime(455): FATAL EXCEPTION: main
05-04 11:14:45.879: ERROR/AndroidRuntime(455): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.xls/com.xls.ExcelActivity}: java.lang.NullPointerException
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1748)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1764)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.ActivityThread.access$1500(ActivityThread.java:122)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1002)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.os.Looper.loop(Looper.java:132)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.ActivityThread.main(ActivityThread.java:4025)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at java.lang.reflect.Method.invokeNative(Native Method)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at java.lang.reflect.Method.invoke(Method.java:491)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at dalvik.system.NativeStart.main(Native Method)
05-04 11:14:45.879: ERROR/AndroidRuntime(455): Caused by: java.lang.NullPointerException
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at com.xls.ExcelActivity.onCreate(ExcelActivity.java:26)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1712)
05-04 11:14:45.879: ERROR/AndroidRuntime(455):     ... 11 more
</pre>

<p>Can someone help solve it ?</p>
