fix(back): only try to insert activities for groups

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/activity/participant.ex

@@ -22,6 +22,10 @@ defmodule Mobilizon.Service.Activity.Participant do
     event = Events.get_event!(event_id)
     subject = Keyword.fetch!(options, :subject)
 
+    if is_nil(event.attributed_to_id) do
+      # No activity for non-group events
+      {:ok, nil}
+    else
       ActivityBuilder.enqueue(:build_activity, %{
         "type" => "event",
         "subject" => subject,
@@ -37,6 +41,7 @@ defmodule Mobilizon.Service.Activity.Participant do
         "inserted_at" => DateTime.utc_now()
       })
     end
+  end
 
   @impl Activity
   def insert_activity(_, _), do: {:ok, nil}