Adding media library field on any entity and making it as required. Create the entity and submit the form without having value in media library field and it will show wrong validation message.
Steps to repeat issue:
1. Enable the Media Library and Media modules.
2. Create content type with Media Library field and make it a required field.
3. Create new content type using Media Library field and save the form to display the wrong error message, "This value should not be null"
See screenshot for error message.