Elixir Unknown Registry Error on Pubsub upgrade

Question

I recently upgraded the pubsub dependency in my project following the documentation, but I am getting this error whenever I run the server:

an exception was raised:
** (ArgumentError) unknown registry: Demo.PubSub
 (elixir 1.10.2) lib/registry.ex:1239: Registry.info!/1
 (elixir 1.10.2) lib/registry.ex:920: Registry.register/3
 (phoenix_pubsub 2.0.0) lib/phoenix/pubsub.ex:117: Phoenix.PubSub.subscribe/3
 (phoenix 1.5.8) lib/phoenix/channel/server.ex:420: Phoenix.Channel.Server.init_join/3
 (phoenix 1.5.8) lib/phoenix/channel/server.ex:378: Phoenix.Channel.Server.channel_join/4
 (phoenix 1.5.8) lib/phoenix/channel/server.ex:298: Phoenix.Channel.Server.handle_info/2
 (stdlib 3.7) gen_server.erl:637: :gen_server.try_dispatch/4
 (stdlib 3.7) gen_server.erl:711: :gen_server.handle_msg/6

Here is my application.ex

def start(_type, _args) do
# List all child processes to be supervised
children = [
 Demo.Repo,
 DemoWeb.Endpoint,
 {Demo.ExpireCodesJob, []},
 {Demo.DistributeCodesJob, []},
 {Oban, oban_config()},
 {Phoenix.PubSub, [name: Demo.PubSub, adapter: Phoenix.PubSub.PG2]},
 worker(PlugAttack.Storage.Ets, [Demo.PlugAttack.Storage, [clean_period: 60_000]])
]

Here is my config.ex

# Configures the endpoint
config :demo, DemoWeb.Endpoint,
 url: [host: System.get_env("HOST")],
 secret_key_base: System.get_env("SECRET_KEY_BASE"),
 render_errors: [view: DemoWeb.ErrorView, accepts: ~w(json)],
 pubsub_server: Demo.PubSub,
 check_origin: [...]

I've searched a lot about it, but since I am still a newbie, could not figure out where does it goes wrong. Please help me out.

• Aleksei Matiushkin: Configs look fine; I believe you just need to move {Phoenix.PubSub, name: Demo.PubSub} child on the top of the children list, like right before/after DemoWeb.Endpoint. The error most likely is induced by the internal attempt to use it before it has been started.
• Tab Key: @AlekseiMatiushkin tried this too... not working