[Pleroma.Web.MastodonAPI.MastodonAPIController]: fallback for try_render/4

Better be sure than sorry

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -1215,4 +1215,10 @@ def try_render(conn, renderer, target, params)
       res
     end
   end
+
+  def try_render(conn, _, _, _) do
+    conn
+    |> put_status(501)
+    |> json(%{error: "Can't display this activity"})
+  end
 end