Plugin deletion fails with status code 500

[SilviaZeta]

Bug Description

Deleting a Python plugin fails with status code 500.

To Reproduce

Steps to reproduce the bugs' behaviour:

1. Go to the 'Plugins' page.
2. Register a Python plugin and add it to a project.
3. Open the project in CURATE and use the plugin.
4. Go back to the 'Plugins' page and delete the plugin.
5. Notice the request failing with status code 500 from the console and the dialog not closing.

Expected Behaviour

The request should return a 200, the plugin should be removed from the table and the 'delete' dialog should close.