How do I change the default file manager back to Nautilus?

This did it for me, after I google for a while:

xdg-mime default nautilus.desktop inode/directory application/x-gnome-saved-search

I also typed:

gsettings set org.gnome.desktop.background show-desktop-icons true

I spent AGES trying to find a method that would work. Eventually I found this advice, which is incredibly easy, and worked for me:

Install exo-utils (this package contains the Xfce settings plugin and the utility files for libexo-2-0). Open a terminal and type:

sudo apt install exo-utils

Then run:

exo-preferred-applications

then switch to Utilities tab and select File Manager you prefer.

The file manager choice is handled by a file in ~/.local/share/applications. They are mimeapps.list and mimeinfo.cache, deleting both of those files will reset your file manager to the default, which is Nautilus.