The code below worked for me I used part of the code above inside the button class.
public Button signupB;
public void handleButtonClick (){
try {
FXMLLoader fxmlLoader = new FXMLLoader();
fxmlLoader.setLocation(getClass().getResource("sceneNotAvailable.fxml"));
/*
* if "fx:controller" is not set in fxml
* fxmlLoader.setController(NewWindowController);
*/
Scene scene = new Scene(fxmlLoader.load(), 630, 400);
Stage stage = new Stage();
stage.setTitle("New Window");
stage.setScene(scene);
stage.show();
} catch (IOException e) {
Logger logger = Logger.getLogger(getClass().getName());
logger.log(Level.SEVERE, "Failed to create new Window.", e);
}
}
}