nixos/river: fix display manager error

        environment.systemPackages = optional (cfg.package != null) cfg.package ++ cfg.extraPackages;

        # To make a river session available if a display manager like SDDM is enabled:

        programs.xwayland.enable = mkDefault true;

        services.xserver.displayManager.sessionPackages = optionals (cfg.package != null) [ cfg.package ];

      }

      (import ./wayland-session.nix { inherit lib pkgs; })

    ]);