How to delete all Table records in Sqlite?

I get JSON value from a server and save the data into a database in Sqlite in Android, I create a database and a table with SQLiteOpenHelper class:

public class ActivityTableDBOpenHelper extends SQLiteOpenHelper {

private static final String LOGTAG = "ATDBOpenHelper";

private static final String DATABASE_NAME = "MyDatabase.db";
private static final int DATABASE_VERSION = 1;

public static final String ACTIVITY_TABLE = "'activity'";
public static final String COLUMN_ACTIVITY_ID = "'activityId'";
public static final String COLUMN_USER_ID = "'userId'";
public static final String COLUMN_PROGRAM_ID = "'programId'";
public static final String COLUMN_ORDER = "'order'";
public static final String COLUMN_G_DATE = "'gDate'";
public static final String COLUMN_J_DATE = "'jDate'";

private static final String TABLE_CREATE =
        "CREATE TABLE " + ACTIVITY_TABLE + " (" +
                COLUMN_ACTIVITY_ID + " INTEGER(20), " +
                COLUMN_USER_ID + " INTEGER(20), " +
                COLUMN_PROGRAM_ID + " INTEGER(20), " +
                COLUMN_ORDER + " VARCHAR(200), " +
                COLUMN_G_DATE + " VARCHAR(200), " +
                COLUMN_J_DATE + " VARCHAR(200) " +
                ")";

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

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL(TABLE_CREATE);
    Log.i(LOGTAG, "Activity Table has been created!");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS" + ACTIVITY_TABLE);
    onCreate(db);
}

}

I can successfully insert data into activity table, now I want to delete all records when user click a button, I use this line code and I get error:

SQLiteDatabase database;
database.delete(ActivityTableDBOpenHelper.ACTIVITY_TABLE,null,null); 

this is my activity class:

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


        btnDeleteATData = (Button) findViewById(R.id.btnDeleteATData);


        dataSource = new ActivityTableDataSource(this);
        dataSource.open();


        btnDeleteATData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                database=new SQLiteDatabase();
                  database.delete(ActivityTableDBOpenHelper.ACTIVITY_TABLE,null,null);
            }
        });       
    }

this is error:

java.lang.NullPointerException: Attempt to invoke virtual method 'int android.database.sqlite.SQLiteDatabase.delete(java.lang.String, java.lang.String, java.lang.String[])' on a null object reference
            at com.rastari.salar.salytest.ActivityTableActivity$2.onClick(ActivityTableActivity.java:69)
            at android.view.View.performClick(View.java:4780)
            at android.view.View$PerformClick.run(View.java:19866)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5254)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

how can I delete all records from activity table?

Thank you.