cant show product in article

Hi,

This error message indicates that the $app variable is not set before in the code. However, if you go up about 200 lines in the same function / file, you'll see this lins near the top of the _processProduct function:
$app = JFactory::getApplication();
I've never seen that line return "null", which your PHP complains about.
So either that line has been removed from that area on your end, or you have a strange problem on your website.
I looked a bit online but I couldn't find really any page talking about JFactory::getApplication returning null, and that's quite normal, as that's almost the main function of Joomla.
So I would rather say that the $app = JFactory::getApplication(); has been removed from the plugin file on your website for some reason.
Or maybe that plugin file was not updated from a really old version of the plugin because of access rights issues on the plugin files. At the beginning of the file, you'll have the version number from which the file comes. If you have HikaShop 4.4.1, it should be the same version number. If you have something else, then you likely have an access rights issue on that file. You'll have to check the permissions there and then run the HikaShop update / install to get the latest version of HikaShop and that file and it should hopefully work fine. Or you could simply add the line $app = JFactory::getApplication(); just before the line of the error, but you might end up with other errors down the line.