Check if node is valid

Problem/Motivation

There should be a check if the $node is an instance of Node on the downloadAllFiles().

Steps to reproduce

Create a Url::fromRoute('download_all_files.download_path', ['nid' => NODE_ID, 'field_name' => FIELD_NAME]) where NODE_ID is not valid node id.

I’ve created a patch that solves the problem by checking the node’s validity.