[android help] building ListView with Adapter

There is a main activity named CategoryListActivity in my project. Please see the following code:

public class CategoryListActivity extends Activity implements LoaderCallbacks> {
    private ArrayAdapter mListAdapter;
    private ListView listview;
    private Categories categories = new Categories();
    private static final int LOADER_ID_CHECKS = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        listview = (ListView) findViewById(R.id.listview);
        mListAdapter = new ArrayAdapter(this, 
                android.R.layout.simple_list_item_1,  
                categories.categoryArray);
        listview.setOnItemClickListener(mOnItemClickListener);
        listview.setAdapter(mListAdapter);

        getLoaderManager().initLoader(LOADER_ID_CHECKS, null, this);       
    }
    ...
}

public class Categories {
    public List categoryArray;
}
public class Category {
    public int categoryId;
    public String categoryName;
    public List questions; 
}
public class Questions {
    public String question;
    public String answer;
}

I've tried to debug the project and it always stop at listview.setAdapter(mListAdapter). It says source not found When running the project, it gives me the following error:

04-25 20:45:01.106: D/AndroidRuntime(19891): Shutting down VM

04-25 20:45:01.106: W/dalvikvm(19891): threadid=1: thread exiting with uncaught exception (group=0x41760930)

04-25 20:45:01.106: E/AndroidRuntime(19891): FATAL EXCEPTION: main

04-25 20:45:01.106: E/AndroidRuntime(19891): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.flashcard/com.example.flashcard.CategoryListActivity}: java.lang.NullPointerException

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.ActivityThread.access$600(ActivityThread.java:141)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.os.Handler.dispatchMessage(Handler.java:99)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.os.Looper.loop(Looper.java:137)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.ActivityThread.main(ActivityThread.java:5041)

04-25 20:45:01.106: E/AndroidRuntime(19891): at java.lang.reflect.Method.invokeNative(Native Method)

04-25 20:45:01.106: E/AndroidRuntime(19891): at java.lang.reflect.Method.invoke(Method.java:511)

04-25 20:45:01.106: E/AndroidRuntime(19891): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)

04-25 20:45:01.106: E/AndroidRuntime(19891): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)

04-25 20:45:01.106: E/AndroidRuntime(19891): at dalvik.system.NativeStart.main(Native Method)

04-25 20:45:01.106: E/AndroidRuntime(19891): Caused by: java.lang.NullPointerException

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:330)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.widget.ListView.setAdapter(ListView.java:462)

04-25 20:45:01.106: E/AndroidRuntime(19891): at com.example.flashcard.CategoryListActivity.onCreate(CategoryListActivity.java:58)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.Activity.performCreate(Activity.java:5104)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)

04-25 20:45:01.106: E/AndroidRuntime(19891): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)

04-25 20:45:01.106: E/AndroidRuntime(19891): ... 11 more

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