symfony 4 configuration default value ( no file ) - symfony

Question

I'm trying to create a default configuration for my bundle. I created the bundle, bundle extension and configuration class. Inside the configuration class, I define a couple of keys with a default value. No yaml files are created. When I try to dump the configuration, I got the error:
"The extension with alias "foxtrot_alpha_users" does not have configuration."
But if I create the matching yaml file, and define at least one of the keys, the other key takes the default value as stated in the configuration class the value can be overridden.
Is it possible to define a default configuration with no yaml file at all?
Bundle class:
class UsersBundle extends Bundle
{
/**
* this is to have a custom alias
*
* #return void
*/
public function getContainerExtension()
{
return( new UsersExtension() );
}
}
Extension class:
class UsersExtension extends Extension
{
public function load(array $configs, ContainerBuilder $container)
{
$configuration = new Configuration();
$config = $this->processConfiguration($configuration, $configs);
foreach($config as $key => $value)
{
$parameterKey = $this->getAlias().'.'.$key;
$container->setParameter($parameterKey, $value);
}
$container->setParameter('default_password', $config['default_password']);
}
/**
* customized alias
*
* #return string
*/
public function getAlias()
{
return('foxtrot_alpha_users');
}
Configuration class:
class Configuration implements ConfigurationInterface
{
public function getConfigTreeBuilder()
{
$treeBuilder = new TreeBuilder('foxtrot_alpha_users');
$rootNode = $treeBuilder->getRootNode();
$this->addUserParameters( $rootNode );
return $treeBuilder;
}
private function addUserParameters( ArrayNodeDefinition $rootNode )
{
$rootNode
->addDefaultsIfNotSet()
->children()
->scalarNode('default_password')
->defaultValue('248')
->end()
->scalarNode('x')
->defaultValue('1')
->end()
->end()
;
return( $rootNode );
}