Everything you need to solve your problem is in the error message! 
Fehler beim Filtern der Vorlage: Notice: Undefined variable: _helper in
When you say $_helper->productAttribute, you're calling a method on the object inside the $_helper variable. At the top of product/list.phtml (which I assume you meant, since Magento 2 doesn't ship with a catalog/list.phtml) you'll see the following code.
#File: vendor/magento/module-catalog/view/frontend/templates/product/list.phtml
$_helper = $this->helper('Magento\Catalog\Helper\Output');That's where $_helper gets defined.
However, in grid.phtml
#File: vendor/magento//module-backend/view/adminhtml/templates/widget/grid.phtml
There's no such $_helper variable defined. I'd start by trying to define that helper at the top of grid.phtml
$_helper = $this->helper('Magento\Catalog\Helper\Output');However, there's also no $_product defined in grid.phtml either, so you'll need to get a reference to a product object somehow. I assume you have that handled, or if you don't you'll provide folks with enough context about what you're doing so they'll be able to help.
Good luck!