In the job configuration if 

TopicAssociation

is enabled and Jenkins is restarted the option is not set anymore.
The reason is that the

 readResolve() 

function checks the config value

 private transient boolean enableTopicAssociation = Config.DEFAULT_ENABLE_TOPIC_ASSOCIATION; 

(GerritTrigger.java) which is always false. Since this condition is always the case the topicAssociation value in the GerritTrigger class is set to "null" (not set).