我的程序有什么问题？为什么会抛出nullpointerexception？ Android

public class tablecontroller extends ListActivity {
    private int mNoteNumber = 1;
    private Dbadapter mDbHelper;
    changetonextlayout pointer;
    SimpleCursorAdapter notes;

    public tablecontroller(changetonextlayout point){
        pointer = point;
        mDbHelper = new Dbadapter(point);
        mDbHelper.open();
        fillData();
    }

    public void createNote() {
        try{
        String noteName = \"Note \" + mNoteNumber ++;
        mDbHelper.createNote(noteName, \"\");
        fillData();
        }
        catch (Exception e)
        {
        // this is the line of code that sends a real error message to the log
        Log.e(\"ERROR\", \"ERROR IN CODE: \" + e.toString());

        // this is the line that prints out the location in
        // the code where the error occurred.
        e.printStackTrace();
        }
    }

    public void fillData() {
        // Get all of the notes from the database and create the item list
        Cursor c = mDbHelper.fetchAllNotes();
        startManagingCursor(c);

        String[] from = new String[] {Dbadapter.KEY_TITLE };
        int[] to = new int[] { R.id.text1 };

        // Now create an array adapter and set it to display using our row
        SimpleCursorAdapter notes =
            new SimpleCursorAdapter(pointer, R.layout.notes_row, c, from, to);
        try{
        setListAdapter(notes);
        }
        catch (Exception e)
        {
        // this is the line of code that sends a real error message to the log
        Log.e(\"ERROR\", \"ERROR IN CODE: \" + e.toString());

        // this is the line that prints out the location in
        // the code where the error occurred.
        e.printStackTrace();
        }
    }
}

setListAdaper

NullPointerException

public class changetonextlayout extends Activity implements OnClickListener{
        protected Context pointer;
        private tablecontroller controller;

        public void onCreate(Bundle savedInstanceState) {
            try
            {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.main2);
            controller = new tablecontroller(this);

            Button buttonback = (Button) findViewById(R.id.button3);
            buttonback.setOnClickListener(this);

            Button buttonsearch = (Button) findViewById(R.id.button4);
            buttonsearch.setOnClickListener(this);

            Button buttonadd = (Button) findViewById(R.id.button5);
            buttonadd.setOnClickListener(this);
            }
            catch (Exception e)
            {
            // this is the line of code that sends a real error message to the log
            Log.e(\"ERROR\", \"ERROR IN CODE: \" + e.toString());

            // this is the line that prints out the location in
            // the code where the error occurred.
            e.printStackTrace();
            }

        }

        public void onClick(View v) {
            // TODO Auto-generated method stub
            int id = v.getId();

            switch (id)
            {
            case 0x7f060013:
                //action of buttonback
                Intent intent = new Intent();
                setResult(RESULT_OK, intent);
                finish();
                break;
            case 0x7f060014:
                //action of buttonsearch
                break;
            case 0x7f060015:
                //action of buttonadd
                controller.createNote();
                break;

            }
        }
    }

public class Dbadapter {
    public static final String KEY_TITLE = \"title\";
    public static final String KEY_BODY = \"body\";
    public static final String KEY_ROWID = \"_id\";

    private static final String TAG = \"Dbadapter\";
    private DatabaseHelper mDbHelper;
    private SQLiteDatabase mDb;

    /**
     * Database creation sql statement
     */
    private static final String DATABASE_CREATE =
            \"create table notes (_id integer primary key autoincrement, \"
                    + \"title text not null, body text not null);\";

    private static final String DATABASE_NAME = \"data\";
    private static final String DATABASE_TABLE = \"notes\";
    private static final int DATABASE_VERSION = 2;

    private final Context mCtx;

    private static class DatabaseHelper extends SQLiteOpenHelper {

        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {

            db.execSQL(DATABASE_CREATE);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            Log.w(TAG, \"Upgrading database from version \" + oldVersion + \" to \"
                    + newVersion + \", which will destroy all old data\");
            db.execSQL(\"DROP TABLE IF EXISTS notes\");
            onCreate(db);
        }
    }

    /**
     * Constructor - takes the context to allow the database to be
     * opened/created
     * 
     * @param ctx the Context within which to work
     */
    public Dbadapter(Context ctx) {
        this.mCtx = ctx;
    }

    /**
     * Open the notes database. If it cannot be opened, try to create a new
     * instance of the database. If it cannot be created, throw an exception to
     * signal the failure
     * 
     * @return this (self reference, allowing this to be chained in an
     *         initialization call)
     * @throws SQLException if the database could be neither opened or created
     */
    public Dbadapter open() throws SQLException {
        mDbHelper = new DatabaseHelper(mCtx);
        mDb = mDbHelper.getWritableDatabase();
        return this;
    }

    public void close() {
        mDbHelper.close();
    }


    /**
     * Create a new note using the title and body provided. If the note is
     * successfully created return the new rowId for that note, otherwise return
     * a -1 to indicate failure.
     * 
     * @param title the title of the note
     * @param body the body of the note
     * @return rowId or -1 if failed
     */
    public long createNote(String title, String body) {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_TITLE, title);
        initialValues.put(KEY_BODY, body);

        return mDb.insert(DATABASE_TABLE, null, initialValues);
    }

    /**
     * Delete the note with the given rowId
     * 
     * @param rowId id of note to delete
     * @return true if deleted, false otherwise
     */
    public boolean deleteNote(long rowId) {

        return mDb.delete(DATABASE_TABLE, KEY_ROWID + \"=\" + rowId, null) > 0;
    }

    /**
     * Return a Cursor over the list of all notes in the database
     * 
     * @return Cursor over all notes
     */
    public Cursor fetchAllNotes() {

        return mDb.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_TITLE,
                KEY_BODY}, null, null, null, null, null);
    }

    /**
     * Return a Cursor positioned at the note that matches the given rowId
     * 
     * @param rowId id of note to retrieve
     * @return Cursor positioned to matching note, if found
     * @throws SQLException if note could not be found/retrieved
     */
    public Cursor fetchNote(long rowId) throws SQLException {

        Cursor mCursor =

                mDb.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
                        KEY_TITLE, KEY_BODY}, KEY_ROWID + \"=\" + rowId, null,
                        null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;

    }

    /**
     * Update the note using the details provided. The note to be updated is
     * specified using the rowId, and it is altered to use the title and body
     * values passed in
     * 
     * @param rowId id of note to update
     * @param title value to set note title to
     * @param body value to set note body to
     * @return true if the note was successfully updated, false otherwise
     */
    public boolean updateNote(long rowId, String title, String body) {
        ContentValues args = new ContentValues();
        args.put(KEY_TITLE, title);
        args.put(KEY_BODY, body);

        return mDb.update(DATABASE_TABLE, args, KEY_ROWID + \"=\" + rowId, null) > 0;
    }
}