单击另一个按钮后无法打开新阶段

Question

我当前的按钮事件处理代码是

public void handle( ActionEvent e )
{
    if ( e.getSource() == menuS )
    {
        Label name = new Label( "Name" );
        Label address = new Label( "Address" );
        Button save = new Button( "Save" );
        GridPane pane = new GridPane();
        ColumnConstraints col = new ColumnConstraints();
        col.setPercentWidth( 50 );
        pane.getColumnConstraints().addAll( col, col );
        pane.add( name, 0, 1, 1, 1 );
        pane.add( address, 0, 2, 1, 1 );
        pane.add( save, 1, 3, 1, 1 );
        pane.add( addressTxt, 1, 2, 1, 1 );
        Scene scene = new Scene( pane );
        Stage stage = new Stage();
        stage.setTitle( "Add Student" );
        stage.setScene( scene );
        stage.show();
    }
    else if ( e.getSource() == menuF )
    {
        Label name = new Label( "Name" );
        Label address = new Label( "Address" );
        Button save = new Button( "Save" );
        GridPane pane = new GridPane();
        ColumnConstraints col = new ColumnConstraints();
        col.setPercentWidth( 50 );
        pane.getColumnConstraints().addAll( col, col );
        pane.add( name, 0, 1, 1, 1 );
        pane.add( address, 0, 2, 1, 1 );
        pane.add( save, 1, 3, 1, 1 );
        Scene scene = new Scene( pane );
        Stage stage = new Stage();
        stage.setTitle( "Add Faculty Member" );
        stage.setScene( scene );
        stage.show();
    }
    else if ( e.getSource() == menuEmp )
    {
        Label name = new Label( "Name" );
        Label address = new Label( "Address" );
        Button save = new Button( "Save" );
        GridPane pane = new GridPane();
        ColumnConstraints col = new ColumnConstraints();
        col.setPercentWidth( 50 );
        pane.getColumnConstraints().addAll( col, col );
        pane.add( name, 0, 1, 1, 1 );
        pane.add( address, 0, 2, 1, 1 );
        pane.add( save, 1, 3, 1, 1 );
        Scene scene = new Scene( pane );
        Stage stage = new Stage();
        stage.setTitle( "Add Student" );
        stage.setScene( scene );
        stage.show();
    }
    else if ( e.getSource() == menuEmp )
    {
        Label name = new Label( "Name" );
        Label address = new Label( "Address" );
        Button save = new Button( "Save" );
        GridPane pane = new GridPane();
        ColumnConstraints col = new ColumnConstraints();
        col.setPercentWidth( 50 );
        pane.getColumnConstraints().addAll( col, col );
        pane.add( name, 0, 1, 1, 1 );
        pane.add( address, 0, 2, 1, 1 );
        pane.add( save, 1, 3, 1, 1 );
        Scene scene = new Scene( pane );
        Stage stage = new Stage();
        stage.setTitle( "Add Student" );
        stage.setScene( scene );
        stage.show();
    }
    else
    {
        Platform.exit();
        System.exit( 0 );
    }

}

我找不到问题。第一个开放给学生，但那里的那些不起作用。有帮助吗？我似乎无法找到任何其他方法来使这个代码功能。