Assuming you have your own CSS for styling the button, this should get things pretty much back to the way they were:
<script type="text/discourse-plugin" version="0.8">
const { iconNode } = require("discourse-common/lib/icon-library");
api.decorateWidget('header-buttons:after', helper => {
if (Discourse.User.current()) {
const createTopic = function() {
const Composer = require('discourse/models/composer').default;
const composerController = Discourse.__container__.lookup('controller:composer');
composerController.open({ action: Composer.CREATE_TOPIC, draftKey: Composer.DRAFT });
}
return helper.h('button#create-new', {
className: "btn fa fa-plus no-text",
title: "Create New Topic",
onclick: createTopic
}, iconNode('plus') );
}
});
</script>