Don't try to fetch OP if none is there.

lib/pleroma/web/mastodon_api/views/status_view.ex

@@ -73,7 +73,11 @@ def render("status.json", %{activity: %{data: %{"object" => object}} = activity}
     created_at = Utils.to_masto_date(object["published"])
 
     # TODO: Add cached version.
-    reply_to = Activity.get_create_activity_by_object_ap_id(object["inReplyTo"])
+    reply_to = if object["inReplyTo"] && object["inReplyTo"] != "" do
+      Activity.get_create_activity_by_object_ap_id(object["inReplyTo"])
+    else
+      nil
+    end
     reply_to_user = reply_to && User.get_cached_by_ap_id(reply_to.data["actor"])
 
     emojis = (activity.data["object"]["emoji"] || [])

test/web/mastodon_api/status_view_test.exs

@@ -54,6 +54,14 @@ test "a note activity" do
     assert status == expected
   end
 
+  test "a reply" do
+    note = insert(:note_activity)
+    user = insert(:user)
+    {:ok, activity} = CommonAPI.post(user, %{"status" => "he", "in_reply_to_status_id" => note.id})
+
+    assert activity.data["object"]["inReplyTo"] == note.data["object"]["id"]
+  end
+
   test "contains mentions" do
     incoming = File.read!("test/fixtures/incoming_reply_mastodon.xml")
     # a user with this ap id might be in the cache.