Quick fix for activities without 'to'.

lib/pleroma/web/twitter_api/twitter_api.ex

@@ -318,7 +318,7 @@ defmodule Pleroma.Web.TwitterAPI.TwitterAPI do
     actor = get_in(activity.data, ["actor"])
     user = User.get_cached_by_ap_id(actor)
     # mentioned_users = Repo.all(from user in User, where: user.ap_id in ^activity.data["to"])
-    mentioned_users = Enum.map(activity.data["to"], fn (ap_id) ->
+    mentioned_users = Enum.map(activity.data["to"] || [], fn (ap_id) ->
       User.get_cached_by_ap_id(ap_id)
     end)
     |> Enum.filter(&(&1))