JavaFX switch scene in Fullscreen

I want to switch the scenes of my JavaFX application in Fullscreen with a "Next"-Button. But if I click on that Button it switches from fullscreen to windowed and back to fullscreen within a second. How can I achieve to avoid that and stay in fullscreen mode?

Some relevant snippets:

Application.java:

public class Application extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("FXMLMain.fxml"));

        Scene scene = new Scene(root);

        stage.setScene(scene);
        stage.show();
        stage.setFullScreen(true);
        stage.setTitle("AppName");

    }

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

}

FXMLMainController.java:

@FXML
private void handleBtnNext(ActionEvent event) throws Exception{
    Stage stage; 
    Parent root;
    if(event.getSource()==btnNext){
        //get reference to the button's stage         
        stage=(Stage) btnNext.getScene().getWindow();
        //load up OTHER FXML document
        root = FXMLLoader.load(getClass().getResource("FXMLOptions.fxml"));
    }
    else{
        stage=(Stage) btnNext.getScene().getWindow();
        root = FXMLLoader.load(getClass().getResource("FXMLMain.fxml"));

    }
    //create a new scene with root and set the stage
    Scene scene = new Scene(root);
    stage.setScene(scene);
    stage.show();
    stage.setFullScreen(true);
}