How to open customer fxml file in MainForm container in Java SE?

Question

working with Java fxml in scenebuilder and have Mainform.fxml file my AppInitialize.java file is like this

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.stage.Stage;

import java.io.IOException;

public class AppInitializer extends Application {

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

    @Override
    public void start(Stage primaryStage) throws IOException {
        primaryStage.setScene(new Scene(FXMLLoader.load(getClass().getResource("./view/MainForm.fxml"))));
        primaryStage.show();

    }
}

my CustomerForm.fxml

 <ImageView disable="true" fitHeight="40.0" fitWidth="40.0" layoutX="40.0" layoutY="201.0" pickOnBounds="true" preserveRatio="true" AnchorPane.leftAnchor="50.0" AnchorPane.rightAnchor="52.0">
                     <image>
                        <Image url="@../asset/customer.png" />
                     </image>
                  </ImageView>

MainFormController.java is

package controller;

import javafx.event.ActionEvent;
import javafx.fxml.FXMLLoader;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.AnchorPane;

import java.io.IOException;

public class MainFormController {
    private AnchorPane container;

    public void customerOnAction(ActionEvent actionEvent) throws IOException {
        AnchorPane pane = FXMLLoader.load(getClass().getResource("CustomerForm.fxml"));
        container.getChildren().setAll(pane);
    }

    public AnchorPane getContainer() {
        return container;
    }

    public void setContainer(AnchorPane container) {
        this.container = container;
    }

    public void customerOnMouseClick(MouseEvent mouseEvent) {
    }
}

but when I click in Customer Button it is not displaying customer form in the MainForm Container. encounting following error message.

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 controller.MainFormController.customerOnAction(MainFormController.java:14)
    ... 58 more

how could I fix this problem?