Responsive 4 page site with ReactJS

This was a fairly easy project but I did have 2 small problems that I had to work around. My main question is about the navbar.

First, I had to set the variable with a useState because the querySelector was responding with a null - is this a good way to set variables? I also had to make another function to close the mobile nav when a link inside was clicked because it would redirect to the correct page but the mobile nav would stay open.

Thanks so much for all critiques + comments!