Problem with adding items to ComboBox in JavaFX

I'm a newbie in JavaFX, trying to start with a windowed application. For start I have a simple ComboBox and I'm filling it with game types, to choose one. Nothing more for start. And there problem begins. According to this answer I have created a model class for entering values in a ComboBox. However, when application starts, there is an error:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException at
com.vojjin.pcreator@1.1/com.vojjin.pcreator.HelloController$1.toString(HelloController.java:23)

Here is the window, with just a Label and ComboBox:

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.effect.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.text.*?>

<Pane prefHeight="250.0" prefWidth="654.0" xmlns="http://javafx.com/javafx/11.0.2" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.vojjin.pcreator.HelloController">
    <padding>
        <Insets bottom="20.0" left="20.0" right="20.0" top="20.0" />
    </padding>
    <Label layoutX="7.0" layoutY="2.0" prefHeight="17.0" prefWidth="94.0" text="Select game type" />
    <ComboBox fx:id="gameTypeCombo" layoutX="7.0" layoutY="20.0" prefWidth="150.0" />
</Pane>

Here is the model class:

package com.vojjin.pcreator.items;

public class GameType {
  private final String name;
  private final String id;

  public GameType(String name, String id) {
    this.name = name;
    this.id = id;
  }
  public String getId() {
    return id;
  }

  public String getName() {
    return name;
  }
}

and Controller class (where I receive an error):

package com.vojjin.pcreator;

import com.vojjin.pcreator.items.GameType;
import javafx.collections.FXCollections;
import javafx.fxml.FXML;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.util.StringConverter;

public class HelloController {
  private String gameType;

  @FXML
  private ComboBox<GameType> gameTypeCombo;

  public void initialize() {
    StringConverter<GameType> t=new StringConverter<>() {
      @Override
      public String toString(GameType object) {
        return object.getName(); //this is the error line
      }

      @Override
      public GameType fromString(String string) {
        return null;
      }
    };

    gameTypeCombo.setConverter(t);

    gameTypeCombo.setItems(FXCollections.observableArrayList(
        new GameType("Arithmo Games", "arithmo"),
        new GameType("iMath Games", "imath")
    ));

    gameTypeCombo.valueProperty().addListener((obs, oldVal, newVal) -> {
      gameType = newVal.getId();
      System.out.println(gameType);
    });
  }

}

Line: return object.getName(); is generating error. That line is what should be displayed in ComboBox. If I put, for example, return "A"; everything is working, except my ComboBox is filled with "A"'s instead of game names. And changing values are working just fine (I'm getting proper getId() value).

Any ideas?