-- HikaShop version -- : 2.3.5
-- Joomla version -- : 3.3.6
When product has no image, default image is not displayed in any view(administration, category listing, product detail).
Also there are notices:
Notice: Trying to get property of non-object in components\com_hikashop\views\product\tmpl\listing_img_title.php on line 24
Notice: Trying to get property of non-object in components\com_hikashop\views\product\tmpl\show_block_img.php on line 64
Notice: Trying to get property of non-object in administrator\components\com_hikashop\views\product\tmpl\listing.php on line 182
Problem is in method hikashopImageHelper::getThumbnail(). In Hikashop 2.3.5 assignment
$extension = strtolower(substr($filename, strrpos($filename, '.') + 1));
were moved from line 437 to the start of method. As $filename is in this case null(product has no image), $extension will be also null and call to method $this->_getImage() will return null instead of object.
Solution is to move assignment in question after line 374.
When you will try to reproduce this bug, don't forget to delete cached thumbnails.