java.lang.IllegalArgumentException: AnchorPane[nowledge-base/tag/javafx">javafx modal-dialog window

I am trying to build a simple appliaction using JavaFX. The problem is when i open a window (modal) the first time it is going well. The second time it's giving me this exception:

java.lang.IllegalArgumentException: AnchorPane[id=klantroot, styleClass=root]is already set as root of another scene

Since i am quite new to this I really don't have an idea on how to fix this. Can someone provide some help please. Thanks a lot in advance.

In my maincontroller I have this code:

private void showModal(Parent view, String title) {
    Stage stage = new Stage();
    Scene scene = new Scene(view);
    stage.setScene(scene);
    stage.initModality(Modality.WINDOW_MODAL);
    stage.initOwner(this.root.getScene().getWindow());
    stage.setTitle(title);
    stage.showAndWait();
}

@FXML
private void handleToevoegenKlant() {
    klantPresenter.setKlant(-1);        
    showModal(klantPresenter.getView(), "Toevoegen klant");
}

In my klantPresenter i have:

@FXML private Parent klantroot;

.....

public Parent getView() {
    return klantroot;
}

@FXML
private void close() {
    Stage stage = (Stage)getView().getScene().getWindow();
    stage.close(); 
}              

The code for KlantView.fxml starts with:

<AnchorPane 
    fx:id="klantroot"
    fx:controller="presenter.KlantPresenter"
    prefHeight="274.0" prefWidth="483.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">