MAGETWO-51402: Fixed JS error when visibility of samples/links fieldset is changed

Author: Bohdan Korablov

app/code/Magento/Downloadable/view/adminhtml/web/js/components/is-downloadable-handler.js

@@ -22,7 +22,7 @@ define([
          * Change visibility for samplesFieldset & linksFieldset based on current statuses of checkbox.
          */
         changeVisibility: function () {
-            if (this.samplesFieldset && this.linksFieldset) {
+            if (this.samplesFieldset() && this.linksFieldset()) {
                 if (this.checked() && !this.disabled()) {
                     this.samplesFieldset().visible(true);
                     this.linksFieldset().visible(true);