+ if (is_null($configKey)) {
+ // Configuration key is null
+ throw new NullPointerException($this, self::EXCEPTION_IS_NULL_POINTER);
+ } elseif ((empty($configKey)) || (!is_string($configKey))) {
+ // Entry is empty
+ throw new InvalidArgumentException('configKey is empty', self::EXCEPTION_CONFIG_KEY_IS_EMPTY);
+ }
+