demo: catch if getContent() return undefined

demo/gadget_demo_json_schema_form.js

@@ -145,6 +145,9 @@
             return g.getContent();
           })
           .push(function (ret) {
+            if (ret === undefined) {
+              ret = {};
+            }
             document.getElementById("json_document_content").textContent =
               JSON.stringify(ret, null, "  ");
           });