timeline: Don't try to use removed subscribers

Because the timeline is essentially asynchronous, it may
try and process events after their subscribers were removed.

Protect against that.

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/576

src/core/gcal-timeline.c

@@ -589,6 +589,13 @@ timeline_source_dispatch (GSource     *source,
       subscriber = queue_data->subscriber;
       event_range = gcal_event_get_range (event);
 
+      /* The subscriber may have been removed already */
+      if (!g_hash_table_contains (self->subscribers, subscriber))
+        {
+          queue_data_free (queue_data);
+          continue;
+        }
+
       switch (queue_data->queue_event)
         {
         case ADD_EVENT: