Delete Physical Instantiation Bug

[ekemeyer]

Describe the bug
When trying to delete a physical instantiation, an error is thrown and the instantiation is not removed, neither from the xml or from the GUI.

To Reproduce
Steps to reproduce the behavior:

1. Go to a record with a physical instantiation
2. Click on physical instantiation thumbnail
3. Press delete
4. See error

Expected behavior
For the physical instantiation data to be removed from both the xml and the gui.

likely related to #146