How to get this working with NixOS and Hyprland?

[caiigee]

Hi, today I decided to setup ReGreet on my NixOS with Hyprland system. I used ReGreet while I had Hyprland on Arch.

I add the following lines to /etc/nixos/configuration.nix:

  services.greetd = {
    enable = true;
    settings = {
      default_session = {
        command = "${pkgs.hyprland}/bin/Hyprland --config /etc/nixos/hypr/hyprland.conf";
      };
      user = "greeter";
    };
  };
  programs.regreet = {
    enable = true;
    settings = {
      background = {
        path = "/etc/nixos/hypr/traveller.png";
      };

      # How the background image covers the screen if the aspect ratio doesn't match 
      # Available values: "Fill", "Contain", "Cover", "ScaleDown" Refer to: 
      # https://docs.gtk.org/gtk4/enum.ContentFit.html NOTE: This is ignored if ReGreet 
      # isn't compiled with GTK v4.8 support.
      fit = "Contain";

      # env = {
        # ENV_VARIABLE = "value";
      # };

      GTK = {
        application_prefer_dark_theme = true;
        cursor_theme_name = "Adwaita";
        font_name = "Cantarell 16";
        icon_theme_name = "Adwaita";
        theme_name = "Adwaita";
      };

      commands = {
        reboot = [ "systemctl" "reboot" ];
        poweroff = [ "systemctl" "poweroff" ];
      };
      
      appearance = {
        greeting_msg = "Welcome back!";
      };
    };
  };

When I ran sudo nixos-rebuild switch and rebooted the system it repeatedly:

1. Shows some logs.
2. Launches Hyprland (I see the wallpaper for a split second).
3. Crashes Hyrpland with the message:

(EE) could not connect to wayland serverME not set, ignoring.n not permitted)---disabling.

I can escape this loop by spamming Ctrl+C and then using Ctrl+Alt+F2 to switch to a different tty and successfully launch Hyprland from it.

So, what is going on here? Did I forget some env variable? Did I write the config incorrectly? What did I do wrong here?