[Sylius] Argument 1 passed to Sylius\Component\Grid\Definition\Filter::setTemplate() must be of the type string, null given fix

 put `grid: <gridname>` into your route like this:

tests:
    path: /tests
    defaults:
        _controller: app.controller.test:indexAction
        _sylius:
            template: tests/index.html.twig
            grid: app_test