Approval Workflow: Content stuck if validation rules change

Description

Steps to reproduce

1. Start the application.
2. Configure Approval Workflow for pages.
3. Make a change in the Start Page.
4. Send the page to review.
5. Stop the application and put [AllowedTypes(typeof(StartPage))] on top of the MainContentArea of the StartPage content type.
6. Start the application again
7. Try to approve the page => an exception is thrown, and it's not possible to put the content back into Draft state nor to approve it.

Expected:
Show a validation message, and have only the ability to decline the review request.

Actual
An exception is thrown, and it's not possible to put the content back into Draft state nor approve it.