Magento 2 – Override Magento\Catalog\Block\Product\View Block

I have developed one custom module and i have tried to override product view block by following these two links Overriding Block in Magento 2
and DI & Extending a Block on Magento 2 but when i hit product view page its gives me 404 page. what i have done so far is below

di.xml

  <?xml version="1.0"?>
   <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
       <preference for="Magento\Catalog\Block\Product\View" type="TT\Helloworld\Block\Myproduct"/>
   </config>

Myproduct.php

<?php
namespace TT\Helloworld\Block;

use Magento\Framework\View\Element\Template;
use Magento\Catalog\Api\ProductRepositoryInterface;
use Magento\Catalog\Model\Product;
class Myproduct extends \Magento\Catalog\Block\Product\View
{

protected $_helper;

protected $_objectManager;

public function __construct(
        \Magento\Catalog\Block\Product\Context $context,
        \Magento\Framework\Url\EncoderInterface $urlEncoder,
        \Magento\Framework\Json\EncoderInterface $jsonEncoder,
        \Magento\Framework\Stdlib\StringUtils $string,
        \Magento\Catalog\Helper\Product $productHelper,
        \Magento\Catalog\Model\ProductTypes\ConfigInterface $productTypeConfig,
        \Magento\Framework\Locale\FormatInterface $localeFormat,
        \Magento\Customer\Model\Session $customerSession,
        ProductRepositoryInterface $productRepository,
        \Magento\Framework\Pricing\PriceCurrencyInterface $priceCurrency,
        array $data = [],
        \TT\Helloworld\Helper\Data $helper  
) {
    parent::__construct($context, $urlEncoder, $jsonEncoder, $string, $productHelper, $productTypeConfig, $localeFormat, $customerSession, $productRepository, $priceCurrency, $data,$helper);

    $this->_helper = $helper;

}

protected function _toHtml()
{
    $this->setModuleName($this->extractModuleName('Magento\Catalog\Block\Product\View'));
    return parent::_toHtml();
}

As per DI & Extending a Block on Magento 2 link i have also included all the parent class construct parameter in Myproduct.php constructer.

anyone one know where i'm wrong? or what is the correct way to override this?