Program: 55875
Project: 55875
Buggy source:
public void setNewMultiPlayerMap(MapLoader mapLoader,IMultiplayerConnector connector){
  this.playerSlotFactory=new HostOfMultiplayerPlayerSlotFactory();
  titleLabel.setText(Labels.getString("String_Node_Str"));
  numberOfPlayersComboBox.setEnabled(false);
  peaceTimeComboBox.setEnabled(false);
  startResourcesComboBox.setEnabled(false);
  startGameButton.setVisible(true);
  setChatVisible(true);
  setStartButtonActionListener(e -> {
  }
);
  IJoiningGame joiningGame=connector.openNewMultiplayerGame(new OpenMultiPlayerGameInfo(mapLoader));
  joiningGame.setListener(new IJoiningGameListener(){
    @Override public void joinProgressChanged(    EProgressState state,    float progress){
    }
    @Override public void gameJoined(    IJoinPhaseMultiplayerGameConnector connector){
      SwingUtilities.invokeLater(() -> {
        initializeChatFor(connector);
        setStartButtonActionListener(e -> {
          connector.startGame();
        }
);
        connector.getPlayers().setListener(changingPlayers -> onPlayersChanges(changingPlayers,connector));
        connector.setMultiplayerListener(new IMultiplayerListener(){
          @Override public void gameIsStarting(          IStartingGame game){
            settlersFrame.showStartingGamePanel(game);
          }
          @Override public void gameAborted(){
            settlersFrame.showMainMenu();
          }
        }
);
      }
);
    }
  }
);
  setCancelButtonActionListener(e -> {
    joiningGame.abort();
    settlersFrame.showMainMenu();
  }
);
  prepareUiFor(mapLoader);
}

Fixed version (ground truth):
public void setNewMultiPlayerMap(MapLoader mapLoader,IMultiplayerConnector connector){
  this.playerSlotFactory=new HostOfMultiplayerPlayerSlotFactory();
  titleLabel.setText(Labels.getString("String_Node_Str"));
  numberOfPlayersComboBox.setEnabled(false);
  peaceTimeComboBox.setEnabled(false);
  startResourcesComboBox.setEnabled(false);
  startGameButton.setVisible(true);
  setChatVisible(true);
  setStartButtonActionListener(e -> {
  }
);
  IJoiningGame joiningGame=connector.openNewMultiplayerGame(new OpenMultiPlayerGameInfo(mapLoader));
  joiningGame.setListener(new IJoiningGameListener(){
    @Override public void joinProgressChanged(    EProgressState state,    float progress){
    }
    @Override public void gameJoined(    IJoinPhaseMultiplayerGameConnector connector){
      SwingUtilities.invokeLater(() -> {
        initializeChatFor(connector);
        setStartButtonActionListener(e -> {
          connector.startGame();
        }
);
        connector.getPlayers().setListener(changingPlayers -> onPlayersChanges(changingPlayers,connector));
        connector.setMultiplayerListener(new IMultiplayerListener(){
          @Override public void gameIsStarting(          IStartingGame game){
            settlersFrame.showStartingGamePanel(game);
          }
          @Override public void gameAborted(){
            settlersFrame.showMainMenu();
          }
        }
);
        onPlayersChanges(connector.getPlayers(),connector);
      }
);
    }
  }
);
  setCancelButtonActionListener(e -> {
    joiningGame.abort();
    settlersFrame.showMainMenu();
  }
);
  prepareUiFor(mapLoader);
}