src/Base/Widget.php
protected function checkAppPersistence()
    {
        if (!isset($this->app) || !$this->app instanceof Reaction\RequestApplicationInterface) {
            $message = sprintf("You must specify \$app (RequestApplicationInterface) parameter for widget '%s'", get_called_class());
            throw new Reaction\Exceptions\InvalidConfigException($message);
        }
    }
protected function checkAppPersistence()
    {
        if (!isset($this->app) || !$this->app instanceof Reaction\RequestApplicationInterface) {
            $message = sprintf("You must specify \$app (RequestApplicationInterface) parameter for widget '%s'", get_called_class());
            throw new Reaction\Exceptions\InvalidConfigException($message);
        }
    }
Check that RequestApplicationInterface is set
@throws Reaction\Exceptions\InvalidConfigException