nullpointer exception trying to save a layout

### Apache NetBeans version

Apache NetBeans 13

### What happened

Trying to save a form that has nested tabbed panes and some panes inside of those. Error message is generated and Netbeans will not save/exit. Error dump follows:
```

java.lang.NullPointerException
at org.netbeans.modules.form.layoutdesign.LayoutPersistenceManager.saveLayout(LayoutPersistenceManager.java:133)
at org.netbeans.modules.form.layoutdesign.LayoutPersistenceManager.saveContainer(LayoutPersistenceManager.java:113)
at org.netbeans.modules.form.layoutdesign.LayoutModel.saveContainerLayout(LayoutModel.java:1193)
at org.netbeans.modules.form.GandalfPersistenceManager.saveLayout(GandalfPersistenceManager.java:3385)
at org.netbeans.modules.form.GandalfPersistenceManager.saveContainer(GandalfPersistenceManager.java:3339)
at org.netbeans.modules.form.GandalfPersistenceManager.saveAnyComponent(GandalfPersistenceManager.java:3312)
at org.netbeans.modules.form.GandalfPersistenceManager.saveContainer(GandalfPersistenceManager.java:3356)
at org.netbeans.modules.form.GandalfPersistenceManager.saveAnyComponent(GandalfPersistenceManager.java:3312)
at org.netbeans.modules.form.GandalfPersistenceManager.saveContainer(GandalfPersistenceManager.java:3356)
at org.netbeans.modules.form.GandalfPersistenceManager.saveAnyComponent(GandalfPersistenceManager.java:3312)
at org.netbeans.modules.form.GandalfPersistenceManager.saveContainer(GandalfPersistenceManager.java:3356)
at org.netbeans.modules.form.GandalfPersistenceManager.saveAnyComponent(GandalfPersistenceManager.java:3312)
at org.netbeans.modules.form.GandalfPersistenceManager.saveForm(GandalfPersistenceManager.java:3194)
at org.netbeans.modules.form.GandalfPersistenceManager.saveForm(GandalfPersistenceManager.java:3116)
at org.netbeans.modules.form.FormEditor.saveFormData(FormEditor.java:397)
at org.netbeans.modules.nbform.FormEditorSupport.saveDocument(FormEditorSupport.java:476)
at org.netbeans.modules.nbform.FormEditorSupport$16.doSave(FormEditorSupport.java:1611)
at org.netbeans.modules.nbform.FormEditorSupport$16.access$1300(FormEditorSupport.java:1582)
at org.netbeans.modules.nbform.FormEditorSupport$16$1.run(FormEditorSupport.java:1594)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:303)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:136)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
[catch] at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

```

### How to reproduce

I don't know how to do this here. It is a .java file that i am trying to save. Nullpointer exception means maybe I missed something?

### Did this work correctly in an earlier version?

Apache NetBeans 13

### Operating System

Ubunutu 20.04

### JDK

JDK 11

### Apache NetBeans packaging

Apache NetBeans binary zip

### Anything else

Everything I try to save, it fails with the above error message.

### Are you willing to submit a pull request?

Yes

### Code of Conduct

Yes

Start at LayoutPersistenceManager.saveLayout on line 133, then follow the saveContainer calls through LayoutModel and GandalfPersistenceManager in the stack trace. Reproduce with the reported form containing nested tabbed panes and identify the state that causes saving to fail. Done means the form saves and exits without the NullPointerException.

Written by the indexing model from the issue text.