An error occurs if you want to check the existence of a specfic parameter, which is not defined. For example:
The message is like:
ContextErrorException: Notice: Undefined offset: 1 in /path/to/symfony/vendor/ezsystems/ezpublish-kernel/eZ/Bundle/EzPublishLegacyBundle/DependencyInjection/Configuration/LegacyConfigResolver.php line 151
In that line it looks like this is only the case if the parameter name contains no peroid. As long as the LegacyConfigResolver is considered to be part of the ChainConfigResolver, this is not the best implementation.