Describe the bug

Selecting a relationship in the Model Browser or right-clicking a relationship in a diagram and selecting "Show in Model Browser" triggers an AssertionError.

To Reproduce

Steps to reproduce the behavior:

1. Click on a relationship in the Model Browser

Time since application startup: 0:00:17
|Traceback (most recent call last):
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\transaction.pyc", line 166, in _transactional
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\transaction.pyc", line 59, in init
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\transaction.pyc", line 114, in _handle
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\transaction.pyc", line 190, in handle
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\application.pyc", line 191, in _transaction_proxy
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\core\eventmanager.pyc", line 77, in handle
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\generic\event.pyc", line 61, in handle
|ExceptionGroup: Error while handling events (1 sub-exception)
└─┬──────────────────────────────╌┄┈
|Traceback (most recent call last):
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\generic\event.pyc", line 57, in handle
| File "C:\opt\dev\gaphor-2.25.1-portable\data_internal\gaphor\services\undomanager.pyc", line 171, in begin_transaction
|AssertionError
└─────────────────────────────╌┄┈

Expected behavior

Select a relationship in either the Model Browser or a diagram and show it in the other view.

Screenshots

N/A

OS

• Linux (Please put in notes the specific distro)
• macOS
• Windows

Version

Version of Gaphor:

Gaphor version: 2.25.1
Operating System: Windows (10)
Display: GdkWin32Display
Python version: 3.12.3
GTK version: 4.14.2
Adwaita version: 1.5.0
GtkSourceView version: 5.12.0
Cairo version: 1.18.0
Pango version: 1.52.2
PyGObject version: 3.48.1
Pycairo version: 1.26.0

Additional information

Add any other context about the problem here.