code: public String getPropertyValue(String propertyName) {
  
          if (m_ownProperties.containsKey(propertyName)) {
              return m_ownProperties.get(propertyName).getEffectiveValue();
          }
          return null;
      }
docs: Returns the property value or null if not set.<p>
  
  @param propertyName the property name
  
  @return the property value