如果第一个jLabel被禁用，java会将图标设置为第二个jLabel

Question

我有两个jLabels。按下按钮时，会将图标设置为JLabel并将其禁用。因此，当按下第二个按钮时，它将检查第一个jLabel是否被禁用，并为下一个jLabel设置图标。以下是我的一个按钮代码（其余按钮的代码相同）：

private void choice1ActionPerformed(java.awt.event.ActionEvent evt) {
choice1.setEnabled(false);
    if(letter1.isEnabled()) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter1.setIcon(ima);
    }
    if(letter1.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter2.setIcon(ima);
    }
    if(letter2.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter3.setIcon(ima);
    }
    if(letter3.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter4.setIcon(ima);
    }
    if(letter4.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter5.setIcon(ima);
    }
    if(letter5.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter6.setIcon(ima);
    }
    if(letter6.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter7.setIcon(ima);
    }
    if(letter7.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter8.setIcon(ima);
    }
    if(letter8.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter9.setIcon(ima);
    }
    if(letter9.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter10.setIcon(ima);
    }
    if(letter10.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter11.setIcon(ima);
    }
    if(letter11.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter12.setIcon(ima);
}
    if(letter12.isEnabled()==false) {
        Icon ima = new ImageIcon(this.getClass().getResource("/replaceletter/lettera2"));
        letter13.setIcon(ima);
    }
}

存在异常（NullPointerException）。并且它无法为jLabel设置图标。我不知道我做错了什么。如有任何建议，将不胜感激

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.ImageIcon.<init>(ImageIcon.java:205)
at Game.GeneticsWord3.choice1ActionPerformed(GeneticsWord3.java:440)
at Game.GeneticsWord3.access$1200(GeneticsWord3.java:13)
at Game.GeneticsWord3$13.actionPerformed(GeneticsWord3.java:244)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at Game.GeneticsWord3.choice1ActionPerformed(GeneticsWord3.java:440)    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6516)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6281)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4872)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:747)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:720)
at java.awt.EventQueue$4.run(EventQueue.java:718)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:717)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)