public function setConfigKeyForFilename(
        string $filename,
        string $configKey,
        $configValue
    ): self {
        if (!isset($this->config[$filename])) {
            $this->config[$filename] = [];
        }
        
        if (!isset($this->configFiles[$filename])) {
            $this->configFiles[$filename] = $filename;
        }
        
        if (is_array($this->config[$filename])) {
            $this->config[$filename][$configKey] = $configValue;
        } else {
            throw new Exception(
                'The config key '.$configKey.' can not be added.',
                $this::ERR_KEY_NOT_ADDED
            );
        }
        
        return $this;
    }