Fix attribute errors when unloading a model

[amolenaar]

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and am following the Contributer guide
• I have read and understand the Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

What is the current behavior?

When closing (flushing) a model (e.g. gaphor/UML/uml2.gaphor) a lot of errors show up:

gaphor.UML.properties ERROR Failed to unlink _stub_7fb1e478f990 from <gaphor.UML.uml2.Class object at 0x7fb1e479cdd0>
Traceback (most recent call last):
  File "/home/arjan/Development/gaphor/gaphor/UML/properties.py", line 549, in unlink
    values = getattr(obj, self._name)
AttributeError: 'Class' object has no attribute '_stub_7fb1e478f990'

Issue Number: N/A

What is the new behavior?

No more errors. This behavior is expected.
The code has been written in a way that expresses the intent more clearly.

Does this PR introduce a breaking change?

• Yes
• No

[danyeaw]

👍