Java，GUI IllegalStateException错误

Question

我知道这已被问过某种状态或形式，我看了别人，但我找不到为什么我收到此错误信息。我对GUI没有太好的研究，这是我正在使用的第一个GUI项目之一。 错误代码：     线程中的异常

"AWT-EventQueue-0" java.lang.IllegalStateException: javax.swing.JLabel[,0,0,0x0,invalid,alignmentX=0.0,alignmentY=0.0,border=,flags=8388608,maximumSize=,minimumSize=,preferredSize=,defaultIcon=,disabledIcon=,horizontalAlignment=LEADING,horizontalTextPosition=TRAILING,iconTextGap=4,labelFor=,text=Label 1,verticalAlignment=CENTER,verticalTextPosition=CENTER] is not attached to a horizontal group
        at javax.swing.GroupLayout.checkComponents(Unknown Source)
        at javax.swing.GroupLayout.prepare(Unknown Source)
        at javax.swing.GroupLayout.layoutContainer(Unknown Source)
        at java.awt.Container.layout(Unknown Source)
        at java.awt.Container.doLayout(Unknown Source)
        at java.awt.Container.validateTree(Unknown Source)
        at java.awt.Container.validateTree(Unknown Source)
        at java.awt.Container.validateTree(Unknown Source)
        at java.awt.Container.validateTree(Unknown Source)
        at java.awt.Container.validateTree(Unknown Source)
        at java.awt.Container.validate(Unknown Source)
        at java.awt.Window.dispatchEventImpl(Unknown Source)
        at java.awt.Component.dispatchEvent(Unknown Source)
        at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
        at java.awt.EventQueue.access$500(Unknown Source)
        at java.awt.EventQueue$3.run(Unknown Source)
        at java.awt.EventQueue$3.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
        at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
        at java.awt.EventQueue$4.run(Unknown Source)
        at java.awt.EventQueue$4.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)

代码：

package GUI;

import java.awt.Component;
import javax.swing.*;
import static javax.swing.GroupLayout.Alignment.*;

import java.awt.*;

public class GUI extends JFrame {
    public GUI() {
        JButton b1 = new JButton("1"),b2=new JButton("2"),b3=new JButton("3"),b4=new JButton("4"),b5=new JButton("5"),b6=new JButton("6");
        JLabel l1= new JLabel("Label 1"),l2= new JLabel("Label 2"),l3=new JLabel("Label 3"),l4=new JLabel("Label 4"),l5=new JLabel("Label 5"),l6=new JLabel("Label 6");
        JPanel pane = new JPanel();
        setTitle("Hello World");
        setSize(500,500);
        setVisible(true);

        GroupLayout layout = new GroupLayout(pane);
        pane.setLayout(layout);
        getContentPane().add(pane);
        layout.setAutoCreateGaps(true);
        layout.setAutoCreateContainerGaps(true);

        layout.setHorizontalGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(LEADING) 
                        .addComponent(b1)
                        .addComponent(b2)
                        .addComponent(b3)
                        )
                .addGroup(layout.createParallelGroup(LEADING)
                        .addComponent(l1)
                        .addComponent(l3)
                        .addComponent(l5)
                        )
                .addGroup(layout.createParallelGroup(LEADING)
                        .addComponent(l2)
                        .addComponent(l4)
                        .addComponent(l6)
                        )
                .addGroup(layout.createParallelGroup(LEADING)
                        .addComponent(b4)
                        .addComponent(b5)
                        .addComponent(b6)
                        ));
        layout.setVerticalGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(BASELINE)
                        .addComponent(b1)
                        .addComponent(l1)
                        .addComponent(l2)
                        .addComponent(b4)
                        )
                .addGroup(layout.createParallelGroup(BASELINE)
                        .addComponent(b2)
                        .addComponent(l3)
                        .addComponent(l4)
                        .addComponent(b5)
                        )
                .addGroup(layout.createParallelGroup(BASELINE)
                        .addComponent(b3)
                        .addComponent(l5)
                        .addComponent(l6)
                        .addComponent(b6)
                        )
                );
    }

}