Definindo o Painel Central do BorderPane no JavaFX criado em FXML (nullPointerException) - java, javafx, fxml

Passei algum tempo agora tentando descobrir onde minha lógica é falha aqui e vi perguntas semelhantes aqui, mas por algum motivo eu continuo recebendo uma nullPointerException:

public class MainWindowController
implements Initializable {


public static final String SCHEDULE_MANAGER = "ScheduleManagerController";
public static final URL SCHEDULE_MANAGER_FXML = ScheduleManagerController.class.getResource("ScheduleManager.fxml");
final SwitchableStackPane customStackPane = new SwitchableStackPane();

@FXML
Button btnScheduleManager; // Value injected by FXMLLoader
BorderPane switchablePane; //Value injected by FXMLLoader


@Override // This method is called by the FXMLLoader when initialization is complete
public void initialize(URL fxmlFileLocation, ResourceBundle resources) {
assert btnScheduleManager != null : "fx:id="ScheduleManager" was not injected: check your FXML file "HOECTool.fxml".";
assert switchablePane != null : "fx:id="mainPane" was not injected: check your FXML file "HOECTool.fxml".";


customStackPane.loadScreen(SCHEDULE_MANAGER, SCHEDULE_MANAGER_FXML);

switchablePane.setCenter(customStackPane);

}

Isso lança uma exceção nullPointer: switchablePane.setCenter (customStackPane);

Respostas:

Ordenado, eu sendo um idiota. Isso foi feito:

 @FXML
Button btnScheduleManager; // Value injected by FXMLLoader
@FXML
BorderPane switchablePane; //Value injected by FXMLLoader