Unable to communicate back with site to check for fatal errors, so the PHP chang

With the plugin enabled, I get the error: Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.

With the plugin disabled, the error goes away.

You may want to see this info on the topic: https://wpengine.com/support/php-editor-errors-wordpress-4-9-higher/

There is also relevant information here: https://wordpress.org/support/topic/cant-edit-main-theme-php-files-after-upgrading-to-4-9/