Double click on connections (relationships) causes error
### Describe the bug

Double clicking on connections (relationships, "Kapcsolatok") causes error:

### To Reproduce

Double clicking on connections (relationships) causes error:

Gaphor version:         2.19.2
Operating System:       Windows (10)
Display:                GdkWin32Display
Python version:         3.11.4
GTK version:            4.10.4
Adwaita version:        1.3.1
GtkSourceView version:  5.8.0
Cairo version:          1.17.8
Pango version:          1.50.14
PyGObject version:      3.44.1
Pycairo version:        1.24.0
pygit2/libgit2 version: 1.12.2  / 1.6.4


Hibák:

Time since application startup: 0:18:54
|Traceback (most recent call last):
|  File "C:\\Home\\Tools\\Gaphor\\gaphor-2.19.2-portable\\data\\gaphor\\ui\\modelbrowser.pyc", line 84, in list_view_activate
|  File "C:\\Home\\Tools\\Gaphor\\gaphor-2.19.2-portable\\data\\gaphor\\ui\\modelbrowser.pyc", line 135, in open_element
|AssertionError
└─────────────────────────────╌┄┈

### Expected behavior
A clear and concise description of what you expected to happen.

### Screenshots

![image](https://github.com/gaphor/gaphor/assets/8182138/5cc12555-82e4-4871-8286-3b9b1754aa6f)

### OS
- [ ] Linux (Please put in notes the specific distro)
- [ ] macOS
- [X] Windows

### Version
Version of Gaphor: 2.19.2


diff --git a/gaphor/ui/modelbrowser.py b/gaphor/ui/modelbrowser.py
index 0125e794a..4c0a3ea4a 100644
--- a/gaphor/ui/modelbrowser.py
+++ b/gaphor/ui/modelbrowser.py
@@ -81,7 +81,8 @@ class ModelBrowser(UIComponent, ActionProvider):
 
         def list_view_activate(list_view, position):
             element = self.selection.get_item(position).get_item().element
-            self.open_element(element)
+            if element:
+                self.open_element(element)
 
         self.tree_view.connect("activate", list_view_activate)
 
['gaphor/ui/modelbrowser.py']