Document Groups doesn't work after upgrade

### Apache NetBeans version

Apache NetBeans 28

### What happened

Document Groups doesn't seem to work after upgrade to NB28 on Windows. The list is available but doesn't open any files.

### Language / Project Type / NetBeans Component

Java Maven Web application project

### How to reproduce

1. In NB26 create a document group.
2. Open some files.
3. Switch to new document group.
4. Upgrade using: https://www.codelerity.com/netbeans/
5. Import settings.
6. You can restart NB just to be sure you have everything.
7. Switch to the document group you created. -> no files are opened.

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

Apache NetBeans 26

### Operating System

Windows 11 25H2

### JDK

Java: 21.0.8; OpenJDK 64-Bit Server VM 21.0.8+9-LTS

### Apache NetBeans packaging

Other

### Anything else

_No response_

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

No

Start by reproducing the Document Groups workflow described for Apache NetBeans 28 on Windows 11, using a group created in NB26 and imported settings. Compare the behavior before and after the upgrade, then trace the Document Groups entry point. Done means switching to the imported group opens its previously saved files.

Written by the indexing model from the issue text.