NPE on Maven if only one of username or password is set

[chanseokoh]

        <configuration>
          <to>
            <image>...</image>
            <auth><username>abc</username></auth>
          </to>
        </configuration>

We have code to give a nice error message If either username or password is set only:

    if (Strings.isNullOrEmpty(auth.getUsername())) {
      eventDispatcher.dispatch(
          LogEvent.warn(
              auth.getUsernamePropertyDescriptor()
                  + " is missing from build configuration; ignoring auth section."));
      return Optional.empty();
    }

However, the checkNotNull inside auth.getUsernamePropertyDescriptor() is failing with the sample config above. Note we set the property descriptor in the JibPluginConfiguration constructor.

  JibPluginConfiguration() {
    to.auth.setPropertyDescriptor("<to><auth>");
    from.auth.setPropertyDescriptor("<from><auth>");
  }

Therefore, when JibPluginConfiguration is created, the property descriptors cannot possible by null. So, I think it may be that if the <auth> section is defined, a new AuthConfiguration instance is created, replacing the one that has property descriptors set by the JibPluginConfiguration?

Caused by: java.lang.NullPointerException
        at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:871)
        at com.google.cloud.tools.jib.maven.JibPluginConfiguration$AuthConfiguration.getPasswordPropertyDescriptor(JibPluginConfiguration.java:59)
        at com.google.cloud.tools.jib.plugins.common.ConfigurationPropertyValidator.getImageCredential(ConfigurationPropertyValidator.java:99)
        at com.google.cloud.tools.jib.maven.BuildImageMojo.execute(BuildImageMojo.java:107)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
        ... 21 more

Haven't tested Gradle.