How to make hide() work (javafx)

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Java

Eclipse

JavaFX

I have created a class called App that extends Application (JavaFX). In it, I have 2 stages that are displaying a few buttons. This App class is instantiated in main then passed as an argument to another class/thread called Controller (a has-a relationship). Once Controller receives an event, it calls app.hideGUI(); the contents of the function are:

public void hideGUI()
{
    handstage.hide();
    stage.hide();
}

Also, in the App class I have the following:

public static void main(String[] args) {
    App app = new App();

    //
    launch(args);
}
public App()
{
    StartLeap();

}

public void StartLeap()
{
    LeapApp.init(true);
    LeapApp.setMode(Mode.INTERACTION_BOX);
}

However, this does not work. It either crashes or simply does not hide the stages. Please advice me on why and how to fix this.

Thank you