Db Handler返回null并且无法创建表？

Question

我有一个数据库类，我用来将一个sql lite数据库连接到一个listview，该类存储了我所有的上下文内容，例如retreieving行，但即使这个简单的getPaitnets也令人心烦这是我的全班

    public class DatabaseHandler extends SQLiteOpenHelper {

        private static final int DATABASE_VERSION = 1;

        private static final String DATABASE_NAME = "CautioManager",
        TABLE_CONTACTS = "patient",
        KEY_ID = "id",
        KEY_NAME = "name",
        KEY_PHONE = "phone",
        KEY_CONTACTPHONE = "contactphone",
        KEY_EMAIL = "email",
        KEY_ADDRESS = "address",
        KEY_IMAGEURI = "imageUri";

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

        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL("CREATE TABLE " + TABLE_CONTACTS + "(" + KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + KEY_NAME + " TEXT," + KEY_PHONE + " TEXT," + KEY_EMAIL + " TEXT," + KEY_ADDRESS + " TEXT," + KEY_IMAGEURI + " TEXT)");
        }

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

            onCreate(db);
        }

        public void createContact(Patients contact) {
            SQLiteDatabase db = getWritableDatabase();

            ContentValues values = new ContentValues();

            values.put(KEY_NAME, contact.getName());
            values.put(KEY_PHONE, contact.getPhone());
            values.put(KEY_CONTACTPHONE, contact.get_contactPhone());

            values.put(KEY_EMAIL, contact.getEmail());
            values.put(KEY_ADDRESS, contact.getAddress());
            values.put(KEY_IMAGEURI, contact.getImageURI().toString());

            db.insert(TABLE_CONTACTS, null, values);
            db.close();
        }

        public Patients getPatientsById(int id) {
            SQLiteDatabase db = getReadableDatabase();

            Cursor cursor = db.query(TABLE_CONTACTS, new String[] { KEY_ID, KEY_NAME, KEY_PHONE, KEY_EMAIL, KEY_ADDRESS, KEY_IMAGEURI }, KEY_ID + "=?", new String[] { String.valueOf(id) }, null, null, null, null );

            if (cursor != null)
                cursor.moveToFirst();

            Patients _patients = new Patients(Integer.parseInt(cursor.getString(0)), cursor.getString(1), cursor.getString(2), cursor.getString(3), cursor.getString(4), cursor.getString(5),  Uri.parse(cursor.getString(6)));
            db.close();
            cursor.close();
            return _patients;
        }

        public void deleteContact(Patients contact) {
            SQLiteDatabase db = getWritableDatabase();
            db.delete(TABLE_CONTACTS, KEY_ID + "=?", new String[] { String.valueOf(contact.getId()) });
            db.close();
        }

        public int getContactsCount() {
            SQLiteDatabase db = getReadableDatabase();
            Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_CONTACTS, null);
            int count = cursor.getCount();
            db.close();
            cursor.close();

            return count;
        }

        public int updatePatients(Patients _Patients) {
            SQLiteDatabase db = getWritableDatabase();

            ContentValues values = new ContentValues();

            values.put(KEY_NAME, _Patients.getName());
            values.put(KEY_PHONE, _Patients.getPhone());
            values.put(KEY_CONTACTPHONE, _Patients.get_contactPhone());

            values.put(KEY_EMAIL, _Patients.getEmail());
            values.put(KEY_ADDRESS, _Patients.getAddress());
            values.put(KEY_IMAGEURI, _Patients.getImageURI().toString());

            int rowsAffected = db.update(TABLE_CONTACTS, values, KEY_ID + "=?", new String[] { String.valueOf(_Patients.getId()) });
            db.close();

            return rowsAffected;
        }

        public List<Patients> getALlPatients() {
            List<Patients> _patients = new ArrayList<Patients>();

            SQLiteDatabase db = getWritableDatabase();
            Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_CONTACTS, null);

            if (cursor.moveToFirst()) {
                do {
                    _patients.add(new Patients(Integer.parseInt(cursor.getString(0)), cursor.getString(1), cursor.getString(2), cursor.getString(3), cursor.getString(4),cursor.getString(5), Uri.parse(cursor.getString(6))));
                }
                while (cursor.moveToNext());
            }
            if (cursor.getCount()==0)
            {

                _patients.add(new Patients(1,"David Buckley","","","","",null));
                _patients.add(new Patients(2,"James Buckley","","","","",null));
            }

            cursor.close();
            db.close();
            return _patients;
        }
    }

然后在我的活动中，我使用它来声明db

的实例

public DatabaseHandler db = new DatabaseHandler(this);

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mNavigationDrawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();

        // Set up the drawer.
        mNavigationDrawerFragment.setUp(
                R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));

  List<Patients> patientsList= new ArrayList<Patients>();




     ArrayAdapter<Patients> adapter = new ArrayAdapter<Patients>(this,android.R.layout.simple_list_item_1,patientsList);

     ListView lv= (ListView) findViewById(R.id.listView);

      lv.setAdapter(adapter);
    }

但是当我编译它并在模拟器上运行它时会返回跟随null对象错误。

对不起，这里的人是错误。

 Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

显示fragment_patient_list的布局文件

 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
         xmlns:tools="http://schemas.android.com/tools"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         tools:context="com.davidbuckley.Cautio.Fragments.PatientsListFragment">

<!-- TODO: Update blank fragment layout -->

 <ListView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/listView"
    android:layout_gravity="center" />
 </FrameLayout>