Android nullpointerexception，我不知道为什么

我得到以下错误：

06-09 06:56:24.675: E/AndroidRuntime(1629): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.gauvion.simpleworkouttracker/com.myapp.MainActivity}: java.lang.NullPointerException

Logcat称其原因是：

06-09 06:56:24.675: E/AndroidRuntime(1629): Caused by: java.lang.NullPointerException
06-09 06:56:24.675: E/AndroidRuntime(1629):     at com.myapp.MainActivity.onCreate(MainActivity.java:26)

以下是MainActivity的代码：

package com.myapp;

import android.app.ListActivity;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.widget.SimpleCursorAdapter;
import android.view.Menu;
import android.widget.Toast;

public class MainActivity extends ListActivity {
    private DayDataSource datasource;
    private SimpleCursorAdapter dataAdapter;
    private Cursor cursor;
    private boolean isEditing = false;
    private Toast toast_deleted;
    private String[] columns = new String[] { MySQLiteHelper.COLUMN_NAME, MySQLiteHelper.COLUMN_DAY };
    private int[] to;

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

        cursor = datasource.fetchAllDays();
        startManagingCursor(cursor);
        to = new int[] { R.id.listitem_day_name, R.id.listitem_day_day };
        dataAdapter = new SimpleCursorAdapter(this, R.layout.listitem_day, cursor, columns, to, 0);
        setListAdapter(dataAdapter);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

}

以下是DayDataSource的代码：

package com.myapp;

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

public class DayDataSource {

    private SQLiteDatabase database;
    private MySQLiteHelper dbHelper;

    public DayDataSource(Context context) {
        dbHelper = new MySQLiteHelper(context);
    }

    public void open() throws SQLException {
        database = dbHelper.getWritableDatabase();
    }

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

    public void createDay(String name, String day) {
        ContentValues values = new ContentValues();
        values.put(MySQLiteHelper.COLUMN_NAME, name);
        values.put(MySQLiteHelper.COLUMN_DAY, day);
        database.insert(MySQLiteHelper.TABLE_DAYS, null, values);
    }

    public void updateDay(long id, String name, String day) {
        ContentValues values = new ContentValues();
        values.put(MySQLiteHelper.COLUMN_NAME, name);
        values.put(MySQLiteHelper.COLUMN_DAY, day);
        database.update(MySQLiteHelper.TABLE_DAYS, values, MySQLiteHelper.COLUMN_ID + " = " + id, null);
    }

    public void deleteDay(long id) {
        database.delete(MySQLiteHelper.TABLE_DAYS, MySQLiteHelper.COLUMN_ID + " = " + id, null);
    }

    public Cursor fetchAllDays() {
        Cursor cursor = database.rawQuery("select " + MySQLiteHelper.COLUMN_ID + ", " + MySQLiteHelper.COLUMN_NAME + ", " + MySQLiteHelper.COLUMN_DAY + " " +
                "from " + MySQLiteHelper.TABLE_DAYS + " " +
                "order by case " + MySQLiteHelper.COLUMN_DAY + " " +
                "when 'Monday' then 0 " +
                "when 'Tuesday' then 1 " +
                "when 'Wednesday' then 2 " +
                "when 'Thursday' then 3 " +
                "when 'Friday' then 4 " +
                "when 'Saturday' then 5 " +
                "when 'Sunday' then 6 " +
                "when 'No Specific Day' then 7 " +
                "end", null);

        if (cursor != null) { cursor.moveToFirst(); }
        return cursor;
    }

} 

任何帮助都将不胜感激。

您正在尝试使用数据源而不进行初始化

cursor = datasource.fetchAllDays();

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在onCreate中，数据源为空。我不得不等12分钟。