activitypub transmogrifier: handle activity lookup failure

with the introduction of MRF, it is possible that an activity may be orphaned from it's parent, if
an activity in the chain was rejected by the active MRF policies.  accordingly, handle this in the
same way as any other orphaned activity.

closes #156

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -30,14 +30,19 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
       when not is_nil(in_reply_to_id) do
     case ActivityPub.fetch_object_from_id(in_reply_to_id) do
       {:ok, replied_object} ->
-        activity = Activity.get_create_activity_by_object_ap_id(replied_object.data["id"])
-
-        object
-        |> Map.put("inReplyTo", replied_object.data["id"])
-        |> Map.put("inReplyToAtomUri", object["inReplyToAtomUri"] || in_reply_to_id)
-        |> Map.put("inReplyToStatusId", activity.id)
-        |> Map.put("conversation", replied_object.data["context"] || object["conversation"])
-        |> Map.put("context", replied_object.data["context"] || object["conversation"])
+        with %Activity{} = activity <-
+               Activity.get_create_activity_by_object_ap_id(replied_object.data["id"]) do
+          object
+          |> Map.put("inReplyTo", replied_object.data["id"])
+          |> Map.put("inReplyToAtomUri", object["inReplyToAtomUri"] || in_reply_to_id)
+          |> Map.put("inReplyToStatusId", activity.id)
+          |> Map.put("conversation", replied_object.data["context"] || object["conversation"])
+          |> Map.put("context", replied_object.data["context"] || object["conversation"])
+        else
+          e ->
+            Logger.error("Couldn't fetch #{object["inReplyTo"]} #{inspect(e)}")
+            object
+        end
 
       e ->
         Logger.error("Couldn't fetch #{object["inReplyTo"]} #{inspect(e)}")