Editing core files like this is a bad way to go about making this change. You basically entered data that Flarum can't handle, even with your minor change. At minimum, you'd need to delete the posts you entered after the change, you may need to scrap your database and start over. And revert your change to the source file, or better yet delete vendor folder and run composer install --prefer-dist --no-dev
If that doesn't work, you'll need to reinstall.
And to make a change like this, it's best to use an extension. You'll need to work with a lot more than the PostValidator to let Flarum handle longer posts. If you want more help, post a topic in the Dev tag.
