In WordPress 5.0, Gutenberg was added as default editor in WordPress. This editor promises to bring a whole new change and experience to users. Nevertheless, after launching, it received plenty of criticism (more than praises) as it caused some problems for users as well as for developers.
Therefore, while Gutenberg is not really stable, it should be disabled if needed.
Otherwise, if you are using other plugins and free or premium themes than you have conflicting problems, you should disable it and go back to the old editor interface.
How to disable Gutenberg using PHP code?
It’s simple to disable Gutenberg, all you need to do is add the following code into the functions.php file in your theme:
add_filter( 'use_block_editor_for_post', '__return_false' );
How to replace Gutenberg with old Classic Editor?
You can also replace Gutenberg editor using this Classic Editor plugin. You just need to install and activate plugin and all done! Cheers!!
This is an official plugin maintained by the WordPress team itself which restores the old WordPress editor for which uses and developers are use to since a long.