private function getPossibleKernelClasses()
    {
        if (empty($this->config['kernel_class'])) {
            return self::$possibleKernelClasses;
        }

        if (!is_string($this->config['kernel_class'])) {
            throw new ModuleException(
                __CLASS__,
                "Parameter 'kernel_class' must have 'string' type.\n"
            );
        }

        return [$this->config['kernel_class']];
    }