if ( existing_plugin == null ){
// create a dummy plugin at version 0.0 to trigger the "upgrade" to the new
// installed version
FailedPlugin dummy_plugin = new FailedPlugin( plugin_id, target_dir );
PluginManager.registerPlugin( dummy_plugin, plugin_id );
final PluginInterface dummy_plugin_interface = manager.getPluginInterfaceByID( plugin_id, false );