If a field has no record in db on a changeset, then the method Tracker_Artifact::getValue($field) will return null. Therefore we have to check the returned value before calling methods on it.

I didn't manage to reproduce the case but at least we should respect the method signature.

This contribution only targets the display of ugroup permissions in project admin page: 
Given an artifact has permission on artifact set to a specific ugroup
And the semantic title is set to a field that is not present in the last changeset of the artifact
When you display the ugroup settings in project admin
Then there should be no fatal error