Over the years, there have been many discussions and debates on whether or not WordPress should have a built-in file editor for themes and plugins. The file editors, while convenient, allow users to easily trigger fatal errors that can be difficult to fix, especially if they don’t have FTP access.
Instead of removing the editors from core, the WordPress development team has enhanced them by adding fatal error protection in WordPress 4.9. When a user accesses the theme or plugin editor for the first time, they’re presented with the following warnings.
If you try to save changes to a file and WordPress detects a fatal error, the change is not saved and a warning message is displayed that explains where the error occurred. Although the changes are rolled back, the code in the editor is not replaced with the original. To replace the code, simply refresh the browser tab.
Taken from WP Tavern, see more WP Tavern.