Describe the bug

When dragging a diagram onto another diagram, (e.g when making a model index) the following error is thrown:
"Element can't be represented on the diagram"

To Reproduce

Steps to reproduce the behavior:

1. create sysml diagram (with frames)
2. create non-sysml diagram
3. drag the icon from sysml diagram onto the non-sysml diagram
4. See error
5. drag non-sysml diagram, onto sysml diagram - see no error.

Gaphor version: 3.0.0
Operating System: Darwin (21.6.0, arm64)
Display: GdkMacosDisplay
Python version: 3.13.0
GTK version: 4.14.4
Adwaita version: 1.5.0
GtkSourceView version: 5.10.0
Cairo version: 1.18.0
Pango version: 1.52.2
PyGObject version: 3.50.0
Pycairo version: 1.27.0

Errors:

Time since application startup: 0:13:15
|Traceback (most recent call last):
| File "/Users/mikekidner/BitBucketRepos/gaphor/gaphor/ui/toolbox.py", line 233, in _flowbox_drag_prepare
| if not child.draggable:
|AttributeError: 'NoneType' object has no attribute 'draggable'
└─────────────────────────────╌┄┈
Time since application startup: 0:13:15
|Traceback (most recent call last):
| File "/Users/mikekidner/BitBucketRepos/gaphor/gaphor/ui/toolbox.py", line 233, in _flowbox_drag_prepare
| if not child.draggable:
|AttributeError: 'NoneType' object has no attribute 'draggable'

Expected behavior

Diagram icons should be able to be represented on diagrams as in 2.27.

Screenshots

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OS

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

Version

Version of Gaphor:3.00

Additional information

Add any other context about the problem here.