Error when user is not signed in : function nil.id/0 is undefined - phoenix-framework

Question

I want to hide edit and delete buttons from users that didnt created topic.
I connected user_id with topics table and that works good.
When user make a new topic his id is in topic user_id.
But now when user is not logged in %{user: nil} i get an error:
function nil.id/0 is undefined
require_auth.ex
def call(conn , _params) do
if conn.assigns[:user] do
conn
else
conn
|> put_flash(:error, "You must be logged in.")
|> redirect(to: ZivotWeb.Router.Helpers.topic_path(conn, :index))
|> halt()
end
set_user.ex
def call(conn, _params) do
user_id = get_session(conn, :user_id)
cond do
user = user_id && Repo.get(User, user_id) ->
assign(conn, :user, user)
true ->
assign(conn, :user, nil)
end
end
I dont know if u need it but there is auth_controller.ex
def callback(%{assigns: %{ueberauth_auth: auth}} = conn ,params) do
user_params = %{token: auth.credentials.token, email: auth.info.email, provider: "github"}
changeset = Zivot.User.changeset(%User{}, user_params)
signin(conn,changeset)
end
def signout(conn,_params) do
conn
|> configure_session(drop: true)
|> put_flash(:info, "you successfully Signed Out!")
|> redirect(to: Routes.topic_path(conn, :index))
end
defp signin(conn, changeset) do
case insert_or_update_user(changeset) do
{:ok, user} ->
conn
|> put_flash(:info, "Welcome back!")
|> put_session(:user_id, user.id)
|> redirect(to: Routes.topic_path(conn, :index))
{:error, _reason} ->
conn
|> put_flash(:error, "Error signing in")
|> redirect(to: Routes.topic_path(conn, :index))
end
end
defp insert_or_update_user(changeset) do
case Repo.get_by(User, email: changeset.changes.email) do
nil ->
Repo.insert(changeset)
user -> {:ok, user}
end
end