Elixir Unknown Registery Error on Pubsub upgrade

Questions & Help Questions
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1

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,
  {Demo.ExpireCodesJob, []},
  {Demo.DistributeCodesJob, []},
  {Oban, oban_config()},
  DemoWeb.Endpoint,
  {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.