@OneToMany property null in Entity after (second) merge

I'm using JPA (with Hibernate) and Gilead in a GWT project. On the server side I have this method and I'm calling this method twice with the same "campaign". On the second call it throws a null pointer exception in line 4 "campaign.getTextAds()"

public List<WrapperTextAd> getTextAds(WrapperCampaign campaign) {
  campaign = em.merge(campaign);
  System.out.println("getting textads for "+campaign.getName());
  for(WrapperTextAd textad: campaign.getTextAds()) {
      //do nothing
  }
  return开发者_StackOverflow new ArrayList<WrapperTextAd>(campaign.getTextAds());
}

The code in WrapperCampaign Entity looks like this

@OneToMany(mappedBy="campaign")
  public Set<WrapperTextAd> getTextAds() {
    return this.textads;
}

Since line 3 doesn't blow, campaign is not null. The for each loop will throw NPE if the collection to be iterated is null (as will trying to initialize a new ArrayList with a null collection parameter). You should guard against this:

if(campaign.getTextAds() != null) {
    for(WrapperTextAd textad: campaign.getTextAds()) {
        //do nothing
    }
}