Program: 19772
Project: 19772
Buggy source:
@Override protected String chooseGroup(){
  logger.log(Level.INFO,"String_Node_Str");
  if (this.getDataModel().getGroupId() == null)   logger.log(Level.INFO,"String_Node_Str" + this.getDataModel().getName() + "String_Node_Str");
 else   logger.log(Level.INFO,"String_Node_Str" + this.getDataModel().getName() + "String_Node_Str"+ this.getDataModel().getEconomicBelief()+ "String_Node_Str"+ getConn().getGroupById(this.getDataModel().getGroupId()).getName());
  logger.log(Level.INFO,"String_Node_Str" + getConn().availableGroups().size());
  String chosenGroup="String_Node_Str";
  if (this.getDataModel().getGroupId() != null) {
    if (groupFounders.contains(this.getId()))     groupFounders.remove(this.getId());
    if (invitationHolders.contains(this.getId()))     invitationHolders.remove(this.getId());
  }
 else   if (this.invitationToGroup != null) {
    logger.log(Level.INFO,"String_Node_Str");
    return this.invitationToGroup;
  }
 else {
    chosenGroup=agentGroupGrouping();
    if (chosenGroup.equals("String_Node_Str"))     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
 else     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
  }
  if (chosenGroup.equals("String_Node_Str")) {
    chosenGroup=freeAgentsGrouping();
    if ((chosenGroup == null))     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
 else     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
  }
  return chosenGroup;
}

Fixed version (ground truth):
@Override protected String chooseGroup(){
  logger.log(Level.INFO,"String_Node_Str");
  if (this.getDataModel().getGroupId() == null)   logger.log(Level.INFO,"String_Node_Str" + this.getDataModel().getName() + "String_Node_Str");
 else   logger.log(Level.INFO,"String_Node_Str" + this.getDataModel().getName() + "String_Node_Str"+ this.getDataModel().getEconomicBelief()+ "String_Node_Str"+ getConn().getGroupById(this.getDataModel().getGroupId()).getName());
  logger.log(Level.INFO,"String_Node_Str" + getConn().availableGroups().size());
  String chosenGroup="String_Node_Str";
  if (this.getDataModel().getGroupId() != null) {
    if (groupFounders.contains(this.getId()))     groupFounders.remove(this.getId());
    if (invitationHolders.contains(this.getId()))     invitationHolders.remove(this.getId());
    return null;
  }
 else   if (this.invitationToGroup != null) {
    logger.log(Level.INFO,"String_Node_Str");
    return this.invitationToGroup;
  }
 else {
    chosenGroup=agentGroupGrouping();
    if (chosenGroup.equals("String_Node_Str"))     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
 else     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
  }
  if (chosenGroup.equals("String_Node_Str")) {
    chosenGroup=freeAgentsGrouping();
    if ((chosenGroup == null))     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
 else     logger.log(Level.INFO,"String_Node_Str" + this.getConn().getAgentById(this.getId()).getName() + "String_Node_Str");
  }
  return chosenGroup;
}