public function getFeatureVariableFromKey($featureFlagKey, $variableKey)
    {
        $featureFlag = $this->getFeatureFlagFromKey($featureFlagKey);
        if ($featureFlag && !($featureFlag->getKey())) {
            return null;
        }

        if (isset($this->_featureFlagVariableMap[$featureFlagKey])
            && isset($this->_featureFlagVariableMap[$featureFlagKey][$variableKey])
        ) {
            return $this->_featureFlagVariableMap[$featureFlagKey][$variableKey];
        }

        $this->_logger->log(
            Logger::ERROR,
            sprintf(
                'No variable key "%s" defined in datafile for feature flag "%s".',
                $variableKey,
                $featureFlagKey
            )
        );
        $this->_errorHandler->handleError(
            new InvalidFeatureVariableException('Provided feature variable is not in datafile.')
        );
        return null;
    }