Add empty array as default for contributors list

app/components/add-proposal/component.js

@@ -5,7 +5,7 @@ import { and, notEmpty } from '@ember/object/computed';
 export default Component.extend({
 
   attributes: null,
-  contributors: null,
+  contributors: Object.freeze([]),
 
   isValidContributor: notEmpty('contributorId'),
   isValidAmount: computed('amount', function() {