从数据库中获取数据和在android中的列表视图中打印时出现问题

Question

获取数据的方法：

public Cursor fetch() {
    String[] columns = new String[] { DbHelper.CONTACTS_COLUMN_ID, DbHelper.ProductName,
    DbHelper.Prodcut_Quantity };
    Cursor cursor = db.query(DbHelper.ORDER_TABLE_NAME, columns, null,null, null, null, null);
    if (cursor != null) {
        cursor.moveToFirst();
    }
    return cursor;
}

并调用actvity

Cursor cursor = databaseHelper.fetch();
String[] from = new String[] { DbHelper.CONTACTS_COLUMN_ID, DbHelper.ProductName,
DbHelper.Prodcut_Quantity  };
int[] to = new int[] { R.id.id, R.id.title, R.id.desc };

adapter = new SimpleCursorAdapter(this, R.layout.activity_view_record, cursor, from, to, 0);
adapter.notifyDataSetChanged();
listView.setAdapter(adapter);

我得到了例外

java.lang.NullPointerException
com.androidbelieve.drawerwithswipetabs.DbHelper.fetch(DbHelper.java:93)
com.androidbelieve.drawerwithswipetabs.Prodculistitem.onCreate(Prodculistitem.java:31)
            at android.app.Activity.performCreate(Activity.java:5231)
            at android.app.Instrumentation.callActivityOnCreate(Instrumen

Null pointer Exception我无法找到原因。我想尝试从数据库中获取数据并将其打印在Listview。

中