Need help in jumping from previous fxml file to new fxml

Question

I am working in JavaFX program. I am trying to jump from my previous fxml file to new fxml file. When I am jumping to my new fxml file, controls are not getting instantiated.

I am getting all these errors:

Caused by: java.lang.NullPointerException
at sample.Controllers.StudentDetailsController.init(StudentDetailsController.java:161)
at sample.Controllers.KeyMapController.studentResponseEvent(KeyMapController.java:159)
at sample.Controllers.KeyMapController.initialize(KeyMapController.java:100)

This is my code to change sceen

try {
    FXMLLoader loader = new FXMLLoader();

    loader.setLocation(getClass().getResource("scenes/studentDetails.fxml"));
    Parent parent = loader.load();
    StudentDetailsController controller = new StudentDetailsController();
    controller.init(testDetails);
    Stage stage = (Stage) ((Node) event.getSource()).getScene().getWindow();
    Scene scene = new Scene(parent);
    stage.setScene(scene);
} catch (IOException e) {
    e.printStackTrace();
}