Исключение при загрузке данных из xml

Я создал JavaFX проект и в контроллере главного окна реализую метод, в котором при нажатии на кнопку должно создаться новое модальное окно, но при при загрузке данных из xml файла выбрасывается исключение. Что не так в моем коде?

package sample;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
public class Main extends Application {
@Override
public void start(Stage primaryStage) throws Exception{
    Parent root = FXMLLoader.load(getClass().getResource("fxml/sample.fxml"));
    primaryStage.setTitle("Телефонная книга");
    primaryStage.setScene(new Scene(root, 600, 300));
    primaryStage.show();
}

public static void main(String[] args) {
    launch(args);
}
}

package sample.controllers;
import javafx.event.ActionEvent;
import javafx.fxml.FXMLLoader;
import javafx.scene.Node;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Modality;
import javafx.stage.Stage;
import java.io.IOException;
public class MainController {

public void removePeople(ActionEvent event) {
    System.out.println("remove");
}
public void addPeople(ActionEvent event) throws IOException {
    Stage stage = new Stage();
    Parent root = FXMLLoader.load(getClass().getResource("fxml/modalWindowAddPeople.fxml"));

    stage.setScene(new Scene(root, 200, 200));
    stage.setTitle("Добавление контакта");
    stage.initModality(Modality.APPLICATION_MODAL);
    stage.setResizable(false);
    stage.initOwner(((Node) event.getSource()).getScene().getWindow());
    stage.show();
}
}

sample.xml
<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.TableColumn?>
<?import javafx.scene.control.TableView?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.VBox?>
<VBox maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity"  minWidth="-Infinity" prefHeight="382.0" prefWidth="602.0"  xmlns="http://javafx.com/javafx/8.0.121" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.controllers.MainController">
....

modalWindowAddPeople.fxml
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<VBox fx:controller="sample.controllers.ModalWindowAddPeople" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="197.0" prefWidth="421.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">

Исключения:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
    at     javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
 .... 
Caused by: java.lang.reflect.InvocationTargetException 
.... 
Caused by: java.lang.NullPointerException: Location is required.
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3207)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3175)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3148)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3124)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3104)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:3097)
at sample.controllers.MainController.addPeople(MainController.java:22)
... 58 more