I am following How to create a nearest node view / Sorting by proximity, which at the fist point says:
Under Advanced, add a contextual filter returning the center entity's ID. For example, if you want to show the view on a node, as a block, select the "Provide default value" radio button and underneath for Type, select "Content ID from URL".
Which contextual filter should I use? I tried Global: Null, but it doesn't seem to be the right one.