Steps to reproduce:

1. Log in to back office.
2. Go to Content / Content structure.
3. Create a new item with richtext field e.g. Article.
4. Add 'h1' paragraph with random string e.g. test.
5. Select text and set formatting block style as 'Highlighted block'.
6. Publish content.

Actual result:

See attachment.

Watch gif please: https://recordit.co/FB6GS1mAng

An error 500 occur while user trying to publish content with formatted block set as 'Highlighted block' in richtext field.

Expected result:

User should be able to publish content with formatted block set as 'Highlighted block'.