quick-add-popover: Only remove existing rows

We can receive the source-changed signal on read-only sources,
and these sources have no row associated. Trying to remove the
non-existing row would yield a warning.

src/gcal-quick-add-popover.c

@@ -536,7 +536,9 @@ on_source_changed (GcalManager         *manager,
   /* If the calendar changed from/to read-only, we add or remove it here */
   if (!gcal_manager_is_client_writable (self->manager, source))
     {
-      gtk_container_remove (GTK_CONTAINER (self->calendars_listbox), row);
+      if (row)
+        gtk_container_remove (GTK_CONTAINER (self->calendars_listbox), row);
+
       return;
     }
   else if (!row)