code: private String getValue(String key, String previousValue) {
          final String value;
          final String valueUpdated = toSave.getProperty(key);
          if (valueUpdated == null) {
              value = previousValue;
          } else {
              value = valueUpdated;
          }
          return value;
      }
docs: Get the value of the property.<br/>
  If the value to save is null, return the previous value (enable/disable mode).
  
  @param key The key of the property
  @param previousValue The previous value
  @return The value of the property