Mrthdrb

app/code/Test/Demo/Block/Widget/Bestsellerdproduct.php

namespace TestDemoBlockWidget;
class Bestsellerdproduct extends MagentoFrameworkViewElementTemplate implements MagentoWidgetBlockBlockInterface

 protected $_template = 'widget/bestsellerdproduct.phtml';

 /**
 * Default value for products count that will be shown
 */
 const DEFAULT_PRODUCTS_COUNT = 10;
 const DEFAULT_IMAGE_WIDTH = 150;
 const DEFAULT_IMAGE_HEIGHT = 150;
 /**
 * Products count
 *
 * @var int
 */
 protected $_productsCount;
 /**
 * @var MagentoFrameworkAppHttpContext
 */
 protected $httpContext;
 protected $_resourceFactory;
 /**
 * Catalog product visibility
 *
 * @var MagentoCatalogModelProductVisibility
 */
 protected $_catalogProductVisibility;

 /**
 * Product collection factory
 *
 * @var MagentoCatalogModelResourceModelProductCollectionFactory
 */
 protected $_productCollectionFactory;

 /**
 * Image helper
 *
 * @var MagentoCatalogHelperImage
 */
 protected $_imageHelper;
 /**
 * @var MagentoCheckoutHelperCart
 */
 protected $_cartHelper;
 /**
 * @param Context $context
 * @param MagentoCatalogModelResourceModelProductCollectionFactory $productCollectionFactory
 * @param MagentoCatalogModelProductVisibility $catalogProductVisibility
 * @param MagentoFrameworkAppHttpContext $httpContext
 * @param array $data
 */
 public function __construct(
 MagentoCatalogBlockProductContext $context,
 MagentoReportsModelResourceModelReportCollectionFactory $resourceFactory,
 MagentoReportsModelGroupedCollectionFactory $collectionFactory,
 MagentoReportsHelperData $reportsData,
 array $data = []
 ) 
 $this->_resourceFactory = $resourceFactory;
 $this->_collectionFactory = $collectionFactory;
 $this->_reportsData = $reportsData;
 $this->_imageHelper = $context->getImageHelper();
 $this->_cartHelper = $context->getCartHelper();
 parent::__construct($context, $data);
 
 /**
 * Image helper Object
 */
 public function imageHelperObj()
 return $this->_imageHelper;
 
 /**
 * get featured product collection
 */
 public function getBestsellerProduct()
 $limit = $this->getProductLimit(); 
 $resourceCollection = $this->_resourceFactory->create('MagentoSalesModelResourceModelReportBestsellersCollection');
 $resourceCollection->setPageSize($limit);
 return $resourceCollection;
 

 /**
 * Get the configured limit of products
 * @return int
 */
 public function getProductLimit() 
 if($this->getData('productcount')=='')
 return DEFAULT_PRODUCTS_COUNT;
 
 return $this->getData('productcount');
 
 /**
 * Get the widht of product image
 * @return int
 */
 public function getProductimagewidth() 
 if($this->getData('imagewidth')=='')
 return DEFAULT_IMAGE_WIDTH;
 
 return $this->getData('imagewidth');
 
 /**
 * Get the height of product image
 * @return int
 */
 public function getProductimageheight() 
 if($this->getData('imageheight')=='')
 return DEFAULT_IMAGE_HEIGHT;
 
 return $this->getData('imageheight');
 
 /**
 * Get the add to cart url
 * @return string
 */
 public function getAddToCartUrl($product, $additional = [])
 
 return $this->_cartHelper->getAddUrl($product, $additional);
 
 /**
 * Return HTML block with price
 *
 * @param MagentoCatalogModelProduct $product
 * @param string $priceType
 * @param string $renderZone
 * @param array $arguments
 * @return string
 * @SuppressWarnings(PHPMD.NPathComplexity)
 */
 public function getProductPriceHtml(
 MagentoCatalogModelProduct $product,
 $priceType = null,
 $renderZone = MagentoFrameworkPricingRender::ZONE_ITEM_LIST,
 array $arguments = []
 ) 
 if (!isset($arguments['zone'])) 
 $arguments['zone'] = $renderZone;
 
 $arguments['zone'] = isset($arguments['zone'])
 ? $arguments['zone']
 : $renderZone;
 $arguments['price_id'] = isset($arguments['price_id'])
 ? $arguments['price_id']
 : 'old-price-' . $product->getId() . '-' . $priceType;
 $arguments['include_container'] = isset($arguments['include_container'])
 ? $arguments['include_container']
 : true;
 $arguments['display_minimal_price'] = isset($arguments['display_minimal_price'])
 ? $arguments['display_minimal_price']
 : true;
 /** @var MagentoFrameworkPricingRender $priceRender */
 $priceRender = $this->getLayout()->getBlock('product.price.render.default');
 $price = '';
 if ($priceRender) 
 $price = $priceRender->render(
 MagentoCatalogPricingPriceFinalPrice::PRICE_CODE,
 $product,
 $arguments
 );
 
 return $price;
 

app/code/Test/Demo/view/frontend/templates/widget/bestsellerdproduct.phtml

 <?php
 if ($exist = ($this->getBestsellerProduct() && $this->getBestsellerProduct()->getPageSize())) 
 $fet_prodcollection = $this->getBestsellerProduct();
 $productcount = $this->getProductLimit();
 $imagewidth = $this->getProductimagewidth();
 $imageheight = $this->getProductimageheight();
 $mode = 'grid';
 $title = __('Best Sellers');
 $type = 'widget-bestseller-grid';
 $image = 'bestseller_products_content_widget_grid';
 $objectManager = MagentoFrameworkAppObjectManager::getInstance();
 
 ?>
<?php if ($exist):?>
<div class="block widget block-bestseller-products <?php /* @escapeNotVerified */ echo $mode; ?>">
<div class="block-title">
 <strong role="heading" aria-level="2"><?php /* @escapeNotVerified */ echo $title; ?></strong>
</div>
<div class="block-content">
<?php /* @escapeNotVerified */ echo '<!-- ' . $image . '-->' ?>
<div class="products-<?php /* @escapeNotVerified */ echo $mode; ?> <?php /* @escapeNotVerified */ echo $mode; ?>">
 <ol class="product-items owl-carousel <?php /* @escapeNotVerified */ echo $type; ?>" id="bestSellers">
 <?php $iterator = 1; ?>
 <?php foreach ($fet_prodcollection as $item): 
 $_product = $objectManager->create('MagentoCatalogModelProduct')->load($item->getProductId());
 //for avoiding not Not Visible Individually
 if($_product->getVisibility() !='1' && $_product->getIsSalable()) 
 ?> 
 <?php /* @escapeNotVerified */ echo($iterator++ == 1) ? '<li class="product-item">' : '</li><li class="product-item">' ?>
 <div class="product-item-info">
 <?php
 $imagewidth='170'; 
 $imageheight ='170';
 $image_url = $block->imageHelperObj()->init($_product, 'product_base_image')
 ->setImageFile($_product->getFile())
 ->resize($imagewidth,$imageheight)
 ->keepAspectRatio(true)
 ->getUrl(); 
 ?>
 <a href="<?php /* @escapeNotVerified */ echo $_product->getProductUrl() ?>" class="product-item-photo">
 <img src="<?php echo $image_url;?>" alt="<?php echo $this->escapeHtml($_product->getName()) ?>" />
 </a>
 <div class="product-item-details">
 <strong class="product-item-name">
 <a title="<?php echo $block->escapeHtml($_product->getName()) ?>"
 href="<?php /* @escapeNotVerified */ echo $_product->getProductUrl() ?>"
 class="product-item-link">
 <?php echo $block->escapeHtml($_product->getName()) ?>
 </a>
 </strong>
 <?php
 echo $this->getProductPriceHtml($_product, $type);
 ?>
 <div class="product-item-actions">
 <div class="actions-primary">
 <?php if ($_product->isSaleable()): ?>
 <?php if ($_product->getTypeInstance()->hasRequiredOptions($_product)): ?>
 <button class="action tocart primary"
 data-mage-init='"redirectUrl":"url":"<?php /* @escapeNotVerified */ echo $block->getAddToCartUrl($_product) ?>"'
 type="button" title="<?php /* @escapeNotVerified */ echo __('Add to Cart') ?>">
 <span><?php /* @escapeNotVerified */ echo __('Add to Cart') ?></span>
 </button>
 <?php else: ?>
 <?php
 $postDataHelper = $this->helper('MagentoFrameworkDataHelperPostHelper');
 $postData = $postDataHelper->getPostData($block->getAddToCartUrl($_product), ['product' => $_product->getId()]);
 ?>
 <button class="action tocart primary" data-post='<?php /* @escapeNotVerified */ echo $postData; ?>' type="button" title="<?php /* @escapeNotVerified */ echo __('Add to Cart') ?>">
 <span><?php /* @escapeNotVerified */ echo __('Add to Cart') ?></span>
 </button>
 <?php endif; ?>
 <?php else: ?>
 <?php if ($_product->getIsSalable()): ?>
 <div class="stock available"><span><?php /* @escapeNotVerified */ echo __('In stock') ?></span></div>
 <?php else: ?>
 <div class="stock unavailable"><span><?php /* @escapeNotVerified */ echo __('Out of stock') ?></span></div>
 <?php endif; ?>
 <?php endif; ?>
 </div>
 </div>
 </div>
 </div>
 <?php ?>
 <?php echo($iterator == count($fet_prodcollection)+1) ? '</li>' : '' ?>
 <?php endforeach ?>
 </ol>
</div>

<?php endif;?>
</div>
</div>

Its good to join the stock filter in your collection for better performance.
However seeing your code you already have any answer for it.

$_product->getIsSalable()

Check this at top before creating your product div.