Fix: skip objects using placeholders without source

[albanbochsler]

Description

For example if an AliasPlugin is saved to the Clipboard it will cause an AttributeError in the alias_usage_view since the placeholder does not have a valid source.

Checklist

• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines

[fsbraun]

Nice catch! Thanks so much for the PR!

Can you add a test (maybe adding an alias plugin to the clipboard and then getting the view)?

[albanbochsler]

Hi @fsbraun

I've added a usage view testcase for alias saved to clipboard.

[fsbraun]

Great work!