Vivaldi is a great browser, very fast, tons of control, very good for Pravacy and security but at some point, for whatever reason if you are to uninstall Vivaldi and going back to one of the mainstream browsers like Chrome or Firefox, you might encounter errors like below.
This is because you uninstalled Vivaldi and the path is no longer there, however, when you opens a link, it is still looking for Vivaldi.
2 options here, install Vivaldi back or select the alternative browser. update-alternatives x-www-browsers to list the alternatives and select the current choice.
just my side note.