Update

lib/nulla_web/controllers/note_controller.ex

@@ -5,22 +5,40 @@ defmodule NullaWeb.NoteController do
   alias Nulla.Models.Note
 
   def show(conn, %{"username" => username, "id" => id}) do
-    accept = List.first(get_req_header(conn, "accept"))
-    note = Note.get_note(id) |> Repo.preload([:actor, :media_attachments])
+    case Integer.parse(id) do
+      {int_id, ""} ->
+        note = Note.get_note(int_id) |> Repo.preload([:actor, :media_attachments])
 
-    if username != note.actor.preferredUsername do
-      conn
-      |> put_status(:not_found)
-      |> json(%{error: "Not Found"})
-      |> halt()
-    end
+        cond do
+          is_nil(note) ->
+            conn
+            |> put_status(:not_found)
+            |> json(%{error: "Not Found"})
+            |> halt()
 
-    if accept in ["application/activity+json", "application/ld+json"] do
-      conn
-      |> put_resp_content_type("application/activity+json")
-      |> json(ActivityPub.note(note))
-    else
-      render(conn, :show, note: note, layout: false)
+          username != note.actor.preferredUsername ->
+            conn
+            |> put_status(:not_found)
+            |> json(%{error: "Not Found"})
+            |> halt()
+
+          true ->
+            accept = List.first(get_req_header(conn, "accept"))
+
+            if accept in ["application/activity+json", "application/ld+json"] do
+              conn
+              |> put_resp_content_type("application/activity+json")
+              |> json(ActivityPub.note(note))
+            else
+              render(conn, :show, note: note, layout: false)
+            end
+        end
+
+      _ ->
+        conn
+        |> put_status(:not_found)
+        |> json(%{error: "Not Found"})
+        |> halt()
     end
   end
 end