JavaFX - creating a new window on button clicked(using Scene Builder)

Question

I want to pop a new window up by clicking a button. since I am using scene builder, my project has main.java and controller.java. and i have two fxml files named Interface.fxml(which primarily appears) and Popup.fxml.

I searched a lot but all the questions and answers were about doing it in main.java not in controller.java. I read about referencing stage and scene in other class but I have failed in implementing.

Here is what i have wrote.

import ...
public class Main extends Application {
  @Override
  public void start(Stage primaryStage) {
    try {
      Parent root = FXMLLoader.load(getClass().getResource("Interface.fxml"));
      Scene scene = new Scene(root);
      primaryStage.setTitle("Calculator");
      primaryStage.setScene(scene);
      primaryStage.show();
    } catch(Exception e) {
      e.printStackTrace();
    }
  }

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

code above is main.java and

@FXML
private void onButtonClicked(ActionEvent ae) {
  try {
    FXMLLoader loader=new FXMLLoader(getClass().getResource("Popup.fxml"));
    loader.setRoot(this);
    loader.setController(this);

    loader.load();

  } catch(Exception e) {
    e.printStackTrace();
  }
}

Code above is in controller.java.

I want to know how to create a new window in a controller.java. Please help me out! (if additional part is needed, please let me know.)