Try to handle misconfiguration scenarios gracefully

lib/pleroma/web/activity_pub/mrf/follow_bot_policy.ex

@@ -1,34 +1,49 @@
 defmodule Pleroma.Web.ActivityPub.MRF.FollowBotPolicy do
   @behaviour Pleroma.Web.ActivityPub.MRF
+  alias Pleroma.Config
   alias Pleroma.User
   alias Pleroma.Web.CommonAPI
   require Logger
 
   @impl true
   def filter(message) do
+    with follower_nickname <- Config.get([:mrf_follow_bot, :follower_nickname]),
+         %User{} = follower <- User.get_cached_by_nickname(follower_nickname),
+         %{"type" => "Create", "object" => %{"type" => "Note"}} <- message do
+      try_follow(follower, message)
+    else
+      nil ->
+        Logger.warn(
+          "#{__MODULE__} skipped because of missing :mrf_follow_bot, :follower_nickname configuration or the account
+            does not exist."
+        )
+
+        {:ok, message}
+
+      _ ->
+        {:ok, message}
+    end
+  end
+
+  defp try_follow(follower, message) do
     Task.start(fn ->
-      follower_nickname = Pleroma.Config.get([:mrf_follow_bot, :follower_nickname])
+      to = Map.get(message, "to", [])
+      cc = Map.get(message, "cc", [])
+      actor = [message["actor"]]
 
-      with %User{} = follower <- User.get_cached_by_nickname(follower_nickname),
-           %{"type" => "Create", "object" => %{"type" => "Note"}} <- message do
-        to = Map.get(message, "to", [])
-        cc = Map.get(message, "cc", [])
-        actor = [message["actor"]]
+      Enum.concat([to, cc, actor])
+      |> List.flatten()
+      |> User.get_all_by_ap_id()
+      |> Enum.each(fn user ->
+        Logger.info("Checking if #{user.nickname} can be followed")
 
-        Enum.concat([to, cc, actor])
-        |> List.flatten()
-        |> User.get_all_by_ap_id()
-        |> Enum.each(fn user ->
-          Logger.info("Checking if #{user.nickname} can be followed")
-
-          with false <- User.following?(follower, user),
-               false <- user.locked,
-               false <- (user.bio || "") |> String.downcase() |> String.contains?("nobot") do
-            Logger.info("Following #{user.nickname}")
-            CommonAPI.follow(follower, user)
-          end
-        end)
-      end
+        with false <- User.following?(follower, user),
+             false <- user.locked,
+             false <- (user.bio || "") |> String.downcase() |> String.contains?("nobot") do
+          Logger.info("Following #{user.nickname}")
+          CommonAPI.follow(follower, user)
+        end
+      end)
     end)
 
     {:ok, message}