Accept strings for selectors in CssRule

src/css_composer/model/CssRule.js

@@ -1,7 +1,7 @@
 import { map } from 'underscore';
 import Backbone from 'backbone';
 import Styleable from 'domain_abstract/model/Styleable';
-import { isEmpty, forEach } from 'underscore';
+import { isEmpty, forEach, isString } from 'underscore';
 import Selectors from 'selector_manager/model/Selectors';
 import { isEmptyObj } from 'utils/mixins';
 
@@ -74,6 +74,8 @@ export default Backbone.Model.extend(Styleable).extend({
       sels = [...sels.models];
     }
 
+    sels = isString(sels) ? [sels] : sels;
+
     if (Array.isArray(sels)) {
       const res = sels.filter(i => i).map(i => (sm ? sm.add(i) : i));
       sels = new Selectors(res);