Add destroy method to CssComposer module

src/css_composer/index.js

@@ -480,6 +480,14 @@ export default () => {
      */
     render() {
       return rulesView.render().el;
+    },
+
+    destroy() {
+      rules.reset();
+      rules.stopListening();
+      rulesView.remove();
+      [em, rules, rulesView].forEach(i => (i = null));
+      c = {};
     }
   };
 };

src/editor/model/Editor.js

@@ -766,7 +766,7 @@ export default Backbone.Model.extend({
     } = this.attributes;
     this.stopDefault();
     DomComponents.clear();
-    CssComposer.clear();
+    CssComposer.destroy();
     UndoManager.clear().removeAll();
     Panels.getPanels().reset();
     Canvas.destroy();