When changing permissions on a permission scheme we get an error

Description
When modifying permissions within a permission scheme, we encounter a 500 error; however, despite the error, the changes are successfully saved.

Pre-conditions

Have a permission scheme 

How to reproduce

1. Go to Settings >> Apps >> [Xporter] Permission Schemes
2. Choose a Scheme and go to Permissions
3. Add an Issue type
4. Click confirm button

Actual result/Attachments

We get an error "Failed to update Permission Scheme." but the changes are saved

Expected result

We get a success message saying "Permission Scheme successfully updated."