List Widget UI values cannot be null

openid-connect-server/src/main/webapp/resources/js/app.js

@@ -83,7 +83,7 @@
                 model = new URIModel({item:input_value});
             } else {
                 model = new Backbone.Model({item:input_value});
-                model.validate = function() {};
+                model.validate = function() { if(!this.get("item")) return "value can't be null" };
             }
 
             // if it's valid and doesn't already exist