emacs 26 with xwidget (doesn't work)

gistfile1.txt

{ pkgs, ... }:

let
  emacs26 = pkgs.stdenv.lib.overrideDerivation pkgs.emacs (oldAttrs : {
    version = "26.1RC1";
    src = pkgs.fetchurl {
      url = ftp://alpha.gnu.org/gnu/emacs/pretest/emacs-26.1-rc1.tar.xz;
      sha256 = "6594e668de00b96e73ad4f168c897fe4bca7c55a4caf19ee20eac54b62a05758";
    };
    patches = [];
    withXwidgetsx = true;
  });
in
{
  environment.systemPackages = [
    emacs26
  ];
}