framework/base/Module.php
public static function setInstance($instance)
    {
        if ($instance === null) {
            unset(Yii::$app->loadedModules[get_called_class()]);
        } else {
            Yii::$app->loadedModules[get_class($instance)] = $instance;
        }
    }
public static function setInstance($instance)
    {
        if ($instance === null) {
            unset(Yii::$app->loadedModules[get_called_class()]);
        } else {
            Yii::$app->loadedModules[get_class($instance)] = $instance;
        }
    }
Sets the currently requested instance of this module class.
@param Module|null $instance the currently requested instance of this module class.
If it is `null`, the instance of the calling class will be removed, if any.