Cannot launch Elixir desktop app when MIX_ENV=test

Background

I have Phoenix umbrella application. When inside said application, I can run it without issues if MIX_ENV=prod. However, if I do mix test while MIX_ENV=dev, I get an error.

Problem

The error I have is as follows:

18:39:42.101 [error] ** wx object server {:local, WebInterface} terminating
** Last message in was {:"$gen_cast", {:show, &WebInterface.Endpoint.url/0}}
** When Server state == %Desktop.Window{
  module: nil,
  taskbar: nil,
  frame: {:wx_ref, 35, :wxFrame, []},
  notifications: %{},
  webview: {:wx_ref, 49, :wxWebView, []},
  home_url: &WebInterface.Endpoint.url/0,
  last_url: nil,
  title: "Market Manager",
  rebuild: 0,
  rebuild_timer: {:interval, #Reference<0.2679771817.2635857922.255366>}
}
** Reason for termination ==
** {:badarg,
 [
   {:erlang, :binary_to_existing_atom,
    ["Elixir.WebInterface.Endpoint.HTTP", :utf8],
    [error_info: %{module: :erl_erts_errors}]},
   {:elixir_aliases, :safe_concat, 1,
    [file: ~c"src/elixir_aliases.erl", line: 139]},
   {WebInterface.Endpoint, :url, 0,
    [file: ~c"lib/web_interface/endpoint.ex", line: 4]},
   {Desktop.Window, :prepare_url, 1,
    [file: ~c"lib/desktop/window.ex", line: 666]},
   {Desktop.Window, :handle_cast, 2,
    [file: ~c"lib/desktop/window.ex", line: 604]},
   {:wx_object, :handle_msg, 5, [file: ~c"wx_object.erl", line: 493]},
   {:proc_lib, :init_p_do_apply, 3, [file: ~c"proc_lib.erl", line: 241]}
 ]}

Reading this, I understand the problem comes from my application.ex, namely:

@impl true
  def start(_type, _args) do
    children = [
      Telemetry,
      {Phoenix.PubSub, name: PubSub},
      Endpoint,
      Manager,
      %{
        id: Desktop.Window,
        start:
          {Desktop.Window, :start_link,
           [
             [
               app: :web_interface,
               id: WebInterface,
               title: "Market Manager",
               size: WindowUtils.calculate_window_size(0.6, 0.8),
               menubar: MenuBar,
               icon: "static/images/resized_logo_5_32x32.png",
               url: &WebInterface.Endpoint.url/0    # This line seems to be a problem
             ]
           ]},
        restart: :transient,
        shutdown: 5_000
      }
    ]

    opts = [strategy: :one_for_one, name: WebInterface.Supervisor]

    Supervisor.start_link(children, opts)
  end

After reading more on the subject I changed my config/test.exs to this:

import Config

config :web_interface, WebInterface.Endpoint,
  http: [ip: {127, 0, 0, 1}, port: 4002],
  secret_key_base: "some_key",
  server: true,
  check_origin: false

But this still does not fix the issue.

In contrast, this is my config/prod.exs:

config :web_interface, WebInterface.Endpoint,
  http: [ip: {127, 0, 0, 1}, port: 80],
  cache_static_manifest: "priv/static/cache_manifest.json",
  secret_key_base: "some_key",
  check_origin: false,
  server: true

But the issue still occurs.

Questions

• Why am I having this issue?
• How can I fix it?