Denis Yanchevskiy

Allows you to make post, page, custom post elements required: title, content, featured image.
After installing the plugin, you need to go to the Settings -> DCO Post Validator to specify the required elements.

DCO Post Validator

Allows you to make post, page, custom post elements required: title, content, featured image.

This plugin will add a new meta box to your posts and pages allowing you to mark posts that require users to login. Visitors trying to view the post, will be asked to login. 
<a href="https://i.imgur.com/ghvTt.png" rel="nofollow ugc">See a screenshot</a>

Simple Content Restriction

Ask your visitors to login if the post is marked as visible only to registered users.

DCO Post Validator vs Simple Content Restriction