问NullPointerException at getWriteableDatabase

EN

public class DatabaseHelper extends SQLiteOpenHelper {

public SQLiteDatabase db;

public static final String DATABASE_NAME = "user_database.db";
public static final String DATABASE_TABLE_NAME = "users";
public static final int DATABASE_VERSION = 1;
public static final String USER_ID = "_id";
public static final String USER_NAME = "user_name";
public static final String USER_SURNAME = "user_surname";

private static final String DATABASE_CREATE =
        "CREATE TABLE " +
        DATABASE_TABLE_NAME +
        " ( " +
        USER_ID +
        " INTEGER PRIMARY KEY AUTOINCREMENT, " +
        USER_NAME +
        " TEXT, " +
        USER_SURNAME +
        " TEXT);";

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

    db = this.getWritableDatabase();
}

@Override
public void onCreate(SQLiteDatabase db) 
{
    db.execSQL(DATABASE_CREATE);

    newEntry("Lorem", "Ipsum");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    Log.w(DatabaseHelper.class.getName(),
            "Upgrading database from version " +
                    oldVersion +
                    " to " +
                    newVersion);
    db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE_NAME);
    onCreate(db);
}

public Cursor newEntry(String name, String surname)
{
    ContentValues cv = new ContentValues();
    cv.put(USER_NAME, name);
    cv.put(USER_SURNAME, surname);
    long id = db.insert(DATABASE_TABLE_NAME, null, cv);

    Cursor cursor = db.query(DATABASE_TABLE_NAME, new String[] {USER_ID, USER_NAME, USER_SURNAME}, USER_ID + " = " + id, null, null, null, null);

    return cursor;
}

public Cursor selectAll()
{
    return db.query(DatabaseHelper.DATABASE_TABLE_NAME, new String[] {USER_ID, USER_NAME, USER_SURNAME}, null, null, null, null, null);
}
}

public class Insert extends Activity implements View.OnClickListener {
public DatabaseHelper myHelper;
public Button button3;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_insert);

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

public void onClick(View v)
{
    String name, surname;
    EditText input;
    Cursor cursor;
    Intent intent;

    input = (EditText) findViewById(R.id.editText1);
    name = input.getText().toString();

    input = (EditText) findViewById(R.id.editText2);
    surname = input.getText().toString();

    myHelper = new DatabaseHelper(this);
    cursor = myHelper.newEntry(name, surname);

    if(cursor.moveToFirst())
    {
        Toast.makeText(this, "Entry added.", Toast.LENGTH_LONG).show();
    }
    else
    {
        Toast.makeText(this, "Entry not added.", Toast.LENGTH_LONG).show();
    }

    cursor.close();

    intent = new Intent(this, Main.class);
    startActivity(intent);
}
}

02-15 20:12:31.117: E/AndroidRuntime(2678): FATAL EXCEPTION: main
02-15 20:12:31.117: E/AndroidRuntime(2678): Process: com.example.userdatabase, PID: 2678
02-15 20:12:31.117: E/AndroidRuntime(2678): java.lang.NullPointerException
02-15 20:12:31.117: E/AndroidRuntime(2678):     at com.example.userdatabase.DatabaseHelper.newEntry(DatabaseHelper.java:66)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at com.example.userdatabase.DatabaseHelper.onCreate(DatabaseHelper.java:47)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:252)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at com.example.userdatabase.DatabaseHelper.<init>(DatabaseHelper.java:39)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at com.example.userdatabase.Insert.onClick(Insert.java:38)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.view.View.performClick(View.java:4438)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.view.View$PerformClick.run(View.java:18422)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.os.Handler.handleCallback(Handler.java:733)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.os.Handler.dispatchMessage(Handler.java:95)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.os.Looper.loop(Looper.java:136)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at android.app.ActivityThread.main(ActivityThread.java:5001)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at java.lang.reflect.Method.invokeNative(Native Method)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at java.lang.reflect.Method.invoke(Method.java:515)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
02-15 20:12:31.117: E/AndroidRuntime(2678):     at dalvik.system.NativeStart.main(Native Method)