第二个窗口不会打开fxml，只打开空fxml

Question

Login.Java

package Login;    
public class Login extends Application{
@Override
public void start(Stage stage)throws Exception{
    Parent root = (Parent) FXMLLoader.load(getClass().getResource("Login.fxml"));
    Scene scene = new Scene(root);
    stage.setScene(scene);
    stage.setTitle("Library Management System");
    stage.show();}
public static void main(String[] args){
launch(args);} }  

LoginController.java

package Login;

public class LoginController implements Initializable {
Librarian l1=new Librarian();
@FXML
private JFXTextField username;
@FXML
private JFXPasswordField password;
@FXML
private JFXButton Loginbutton;
@FXML
private JFXButton CancelButton;
@Override
public void initialize(URL url, ResourceBundle rb) {               
}    
@FXML
private void LibrarianLogin(ActionEvent event) throws SQLException {

    if(l1.isLogin(username.getText(),password.getText())){

            Stage stage = (Stage) this.Loginbutton.getScene().getWindow();
            stage.close();
            adminLogin();
}        
}

@FXML
private void handleCancelBtnAction(ActionEvent event) {
}
 public void adminLogin()
{
 try{
     Stage adminstage = new Stage();
     FXMLLoader adminLoader = new FXMLLoader();
     Pane adminroot = (Pane)adminLoader.load(getClass().getResource("/Librarian/admin.fxml").openStream());
     AdminController adminController = (AdminController)adminLoader.getController();
     Scene scene =new Scene(adminroot);
     adminstage.setScene(scene);
     adminstage.setTitle("Admin Dashboard");
     adminstage.show();

 }
 catch (IOException e){
 }
}
} 

按下登录按钮时会发生错误！我不知道该怎么办!!! 当我用空的fxml替换admin.fxml时，当我点击登录按钮时会出现另一个窗口！ 除了这些警告之外，我没有任何错误：

Apr 21, 2018 1:38:07 AM javafx.fxml.FXMLLoader$ValueElement processValue
WARNING: Loading FXML document with JavaFX API of version 9.0.1 by JavaFX 
runtime of version 8.0.161
Apr 21, 2018 1:38:12 AM javafx.fxml.FXMLLoader$ValueElement processValue
WARNING: Loading FXML document with JavaFX API of version 9.0.1 by JavaFX 
runtime of version 8.0.161