fix the variable declaration mistake

2-ui/1-document/07-modifying-document/7-create-object-tree/innerhtml.view/index.html

@@ -29,11 +29,12 @@
 
     function createTreeText(obj) { // standalone recursive function
       let li = '';
+      let ul;
       for (let key in obj) {
         li += '<li>' + key + createTreeText(obj[key]) + '</li>';
       }
       if (li) {
-        let ul = '<ul>' + li + '</ul>'
+        ul = '<ul>' + li + '</ul>'
       }
       return ul || '';
     }