我无法从SQLite db - Android获取ContentProvider信息

Question

public class Cliente扩展了SQLiteOpenHelper {

public Cliente(Context context, String name, CursorFactory factory,
        int version) {
    super(context, name, factory, version);
    // TODO Auto-generated constructor stub
}

private String query = "CREATE TABLE Clientes "+
"(_id INTEGER PRIMARY KEY AUTOINCREMENT,"+
" nombre TEXT, "+
" telefono TEXT, "+
" email TEXT )";



@Override
public void onCreate(SQLiteDatabase db) {
    // TODO Auto-generated method stub
    db.execSQL(query);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // TODO Auto-generated method stub
    db.execSQL("DROP TABLE Clientes");
    db.execSQL(query);
}

}

公共类ClienteProvider扩展了ContentProvider {

public static final Uri CONTENT_URI = Uri.parse("content://com.inkadroid.ejemplocp/Cliente");

//Nombres de columnas

public static final String COL_NOMBRE = "nombre";
public static final String COL_TELEFONO = "telefono";
public static final String COL_EMAIL = "email";


private Cliente db;
private static final String BD_NOMBRE = "BD_Ventas";
private static final int BD_VERSION = 1;
private static final String TABLA_CLIENTES = "Clientes";

//UriMatcher acceso a una tabla o registro especifico
private static final int CLIENTES = 1; 
private static final int CLIENTES_ID = 2; 
private static final UriMatcher uriMatcher;
static {
    uriMatcher = new UriMatcher(UriMatcher.NO_MATCH); 
    uriMatcher.addURI("com.inkadroid.ejmeplocp2", "Clientes", CLIENTES); 
    uriMatcher.addURI("com.inkadroid.ejmeplocp2", "Clientes/#", CLIENTES_ID);
}

@Override
public boolean onCreate() {
    // TODO Auto-generated method stub
    return true;
}

@Override
public Cursor query(Uri uri, String[] projection, String selection,
        String[] selectionArgs, String sortOrder) {
    // TODO Auto-generated method stub
    //Si es una consulta a un ID concreto construimos el WHERE
    String where = selection;

    Log.e("Select", where);

    if(uriMatcher.match(uri) == CLIENTES_ID){

        Log.e("URI", uri+"");
        where = "_id=" + uri.getLastPathSegment();
    }

    SQLiteDatabase access= db.getWritableDatabase();
    Cursor c = access.query(TABLA_CLIENTES, projection, where, selectionArgs, null, null,
    sortOrder);
    Log.e("TABLA_CLIENTES", TABLA_CLIENTES);
    Log.e("where", where);

    return c;

}

}

公共类MainActivity扩展了Activity {

String[] projection = new String[]{"codigo","nombre","telefono","email"};
String selection = "SELECT * FROM Clientes ;";
String[] selectionArgs = null;
String sortOrder = null;


ClienteProvider clienteProvider;

ContentResolver resolver;



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

    Cliente cliente = new Cliente(this, "BD_Ventas", null, 1);
    SQLiteDatabase conexion = cliente.getWritableDatabase();

    for(int i=0; i<10;i++){
        conexion.execSQL("INSERT INTO Clientes (nombre,telefono,email) values ('cliente"+i+"','977915615','cliente@gmail.com')");

    }

    Cursor cursor = resolver.query(ClienteProvider.CONTENT_URI,projection,selection,selectionArgs,sortOrder);


    while (cursor.moveToNext()) {

        do {
            String codigo= cursor.getString(0);
            Log.e("Codigo", codigo);
            String nombre= cursor.getString(1);
            Log.e("nombre", nombre);
            String telefono = cursor.getString(2);
            Log.e("telefono", telefono);
            String email = cursor.getString(3);
            Log.e("email", email);

            String inString = codigo + " " + nombre + " " + telefono +" " + email + " ."; 
            Log.e("Sentencia", inString);
            Toast.makeText(this, inString, Toast.LENGTH_LONG).show();

        } while(cursor.moveToNext());



    }    

    conexion.close();

}

}

01-02 02：01：56.710：E / AndroidRuntime（13717）：java.lang.RuntimeException：无法启动活动ComponentInfo {com.inkadroid.ejemplocp / com.inkadroid.ejemplocp.MainActivity}：java.lang。的NullPointerException