wro4j-core/src/main/java/ro/isdc/wro/manager/factory/ConfigurableWroManagerFactory.java
private Properties getConfigProperties() {
    if (configProperties == null) {
      configProperties = newConfigProperties();
      if (additionalConfigProperties != null) {
        configProperties.putAll(additionalConfigProperties);
      }
    }
    return configProperties;
  }
private Properties getConfigProperties() {
    if (configProperties == null) {
      configProperties = newConfigProperties();
      if (additionalConfigProperties != null) {
        configProperties.putAll(additionalConfigProperties);
      }
    }
    return configProperties;
  }
Use this method rather than accessing the field directly, because it will create a default one if none is provided.