Android에서 sqlite 데이터베이스를 사용하는 NullpointerException

android를 사용하여 재생 목록 개체를 sqlite 데이터베이스에 삽입하려고하는데 데이터베이스를 열 때 NullPointerException이 발생합니다. 내 PlaylistDbAdapter는 다음과 같습니다.

    import java.util.ArrayList;
    import java.util.List;

    import android.content.ContentValues;
    import android.content.Context;
    import android.database.Cursor;
    import android.database.SQLException;
    import android.database.sqlite.SQLiteDatabase;
    import android.database.sqlite.SQLiteOpenHelper;

    public class PlaylistDbAdapter{

// All Static variables
// Database Version
private static final int DATABASE_VERSION = 2;

// Database Name
private static final String DATABASE_NAME = "playlistdb";

// Contacts table name
private static final String TABLE_NAME = "playlists";

// Contacts Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_NAME = "name";
private DatabaseHelper dh;
 private SQLiteDatabase mDb;
private final Context mCtx;

private static class DatabaseHelper extends SQLiteOpenHelper{

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

// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
    String CREATE_PLAYLISTS_TABLE =  "create table playlists (id integer primary key autoincrement, name text not null);";
    db.execSQL(CREATE_PLAYLISTS_TABLE);
}

// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // Drop older table if existed
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);

    // Create tables again
    onCreate(db);
}
}


 public  PlaylistDbAdapter(Context ctx) {
        this.mCtx = ctx;
    }
 public PlaylistDbAdapter open() throws SQLException {
        dh = new DatabaseHelper(mCtx);
        mDb = dh.getWritableDatabase();
        return this;
    }

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

/**
 * All CRUD(Create, Read, Update, Delete) Operations
 * @return 
 */

void addPlaylist(Playlist playlist) {

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, playlist.getName()); // Playlist Name

    // Inserting Row
    mDb.insert(TABLE_NAME, null, values);
    mDb.close(); // Closing database connection
}

// Getting single playlist
Playlist getPlaylist(int id) {
    SQLiteDatabase db = dh.getReadableDatabase();

    Cursor cursor = db.query(TABLE_NAME, new String[] { KEY_ID,
            KEY_NAME}, KEY_ID + "=?",
            new String[] { String.valueOf(id) }, null, null, null);
    if (cursor != null)
        cursor.moveToFirst();

    Playlist playlist = new Playlist (Integer.parseInt(cursor.getString(0)),
            cursor.getString(1));
    // return playlist
    System.out.println(playlist._name);
    return playlist;
}

// Getting All Playlists
public List<Playlist> getAllPlaylists() {
    List<Playlist> contactList = new ArrayList<Playlist>();
    // Select All Query
    String selectQuery = "SELECT  * FROM " + TABLE_NAME;

    SQLiteDatabase db = dh.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);

    // looping through all rows and adding to list
    if (cursor.moveToFirst()) {
        do {
            Playlist playlist = new Playlist();
            playlist.setID(Integer.parseInt(cursor.getString(0)));
            playlist.setName(cursor.getString(1));
            // Adding playlist to list
            contactList.add(playlist);
        } while (cursor.moveToNext());
    }

    // return playlist list
    return contactList;
}

// Updating single playlist
public int updatePlaylist(Playlist playlist) {
    SQLiteDatabase db = dh.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, playlist.getName());

    // updating row
    return db.update(TABLE_NAME, values, KEY_ID + " = ?",
            new String[] { String.valueOf(playlist.getID()) });
}

// Deleting single playlist
public void deletePlaylist(Playlist playlist) {
    SQLiteDatabase db = dh.getWritableDatabase();
    db.delete(TABLE_NAME, KEY_ID + " = ?",
            new String[] { String.valueOf(playlist.getID()) });
    db.close();
}


// Getting playlists Count
public int getPlaylistCount() {
    String countQuery = "SELECT  * FROM " + TABLE_NAME;
    SQLiteDatabase db = dh.getReadableDatabase();
    Cursor cursor = db.rawQuery(countQuery, null);
    cursor.close();
    // return count
    return cursor.getCount();
}

    }

이것은 내 재생 목록 개체입니다

    public class Playlist {


//private variables
int _id;
String _name;

// Empty constructor
public Playlist(){

}
// constructor
public Playlist(int id, String name){
    this._id = id;
    this._name = name;
}

// constructor
public Playlist(String name){
    this._name = name;
}
// getting ID
public int getID(){
    return this._id;
}

// setting id
public void setID(int id){
    this._id = id;
}

// getting name
public String getName(){
    return this._name;
}

// setting name
public void setName(String name){
    this._name = name;
}

    }

그리고 이것은 사용자가 재생 목록의 이름을 선택하는 경고 대화 상자를 사용하여 새 재생 목록을 추가하는 AddPlaylist 클래스입니다.

    import android.os.Bundle;

    import android.text.InputType;
    import android.view.View;
    import android.widget.EditText;
    import android.widget.ImageButton;
    import android.widget.Toast;
    import android.app.Activity;
    import android.app.AlertDialog;
    import android.content.Context;
    import android.content.DialogInterface;
    import android.database.sqlite.SQLiteDatabase;


    public class AddPlaylist extends Activity{
private ImageButton btnAddPlaylist;
private String playlistName = "";
public PlaylistDbAdapter dh;
private SQLiteDatabase db;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);     
    setContentView(R.layout.add_playlist);
    btnAddPlaylist = (ImageButton) findViewById(R.id.btnAddPlaylist);
    btnAddPlaylist.setOnClickListener(new View.OnClickListener(){

        @Override
        public void onClick(View arg0) {
            popUpNewPlaylist();
        }

    });
}
public void popUpNewPlaylist()
{   

    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle("Playlist name");

    // Set up the input
    final EditText input = new EditText(this);
    // Specify the type of input expected; this, for example, sets the input as a password, and will mask the text
    input.setInputType(InputType.TYPE_CLASS_TEXT | InputType.TYPE_CLASS_TEXT);
    builder.setView(input);

    // Set up the buttons
    builder.setPositiveButton("Add", new DialogInterface.OnClickListener() { 
        @Override
        public void onClick(DialogInterface dialog, int which) {
            playlistName = input.getText().toString();
            Playlist tmpPLST = new Playlist();
            tmpPLST.setName(playlistName);
            dh.open();
            //dh.addPlaylist(tmpPLST);
            popupSuccessToast();
        }
    });
    builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
            popupCancelToast();
        }
    });

    builder.show();
}

public void popupSuccessToast()
{
    Context context = getApplicationContext();
    CharSequence text = "Playlist "+playlistName+" Added";
    int duration = Toast.LENGTH_SHORT;

    Toast toast = Toast.makeText(context, text, duration);
    toast.show();
}

public void popupCancelToast()
{
    Context context = getApplicationContext();
    CharSequence text = "Operation canceled";
    int duration = Toast.LENGTH_SHORT;

    Toast toast = Toast.makeText(context, text, duration);
    toast.show();
}


    } 

내 충돌 로그

    04-27 18:43:34.151: E/AndroidRuntime(4338): FATAL EXCEPTION: main
    04-27 18:43:34.151: E/AndroidRuntime(4338): java.lang.NullPointerException
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at                               ca.uqac.powerplayer.AddPlaylist$2.onClick(AddPlaylist.java:58)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at             com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java)
     04-27 18:43:34.151: E/AndroidRuntime(4338):    at    android.os.Handler.dispatchMessage(Handler.java)
     04-27 18:43:34.151: E/AndroidRuntime(4338):    at android.os.Looper.loop(Looper.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at android.app.ActivityThread.main(ActivityThread.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at java.lang.reflect.Method.invokeNative(Native Method)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at java.lang.reflect.Method.invoke(Method.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at dalvik.system.NativeStart.main(Native Method)

감사.