Update README.md

Fix node not defined error in README.md examples

Author: johannalee

README.md

@@ -84,7 +84,7 @@ function* treeWalker(refresh) {
     const isOpened = yield refresh
       ? {
           id,
-          isLeaf: children.length === 0,
+          isLeaf: children?.length === 0,
           isOpenByDefault: true,
           name,
           nestingLevel,
@@ -93,13 +93,13 @@ function* treeWalker(refresh) {
 
     // Basing on the node openness state we are deciding if we need to render
     // the child nodes (if they exist).
-    if (node.children.length !== 0 && isOpened) {
+    if (children?.length !== 0 && isOpened) {
       // Since it is a stack structure, we need to put nodes we want to render
       // first to the end of the stack.
-      for (let i = node.children.length - 1; i >= 0; i--) {
+      for (let i = children?.length - 1; i >= 0; i--) {
         stack.push({
           nestingLevel: nestingLevel + 1,
-          node: node.children[i],
+          node: children[i],
         });
       }
     }
@@ -253,18 +253,18 @@ function* treeWalker(refresh) {
           // object.
           defaultHeight: 30,
           id,
-          isLeaf: children.length === 0,
+          isLeaf: children?.length === 0,
           isOpenByDefault: true,
           name,
           nestingLevel,
         }
       : id;
 
-    if (node.children.length !== 0 && isOpened) {
-      for (let i = node.children.length - 1; i >= 0; i--) {
+    if (children?.length !== 0 && isOpened) {
+      for (let i = children?.length - 1; i >= 0; i--) {
         stack.push({
           nestingLevel: nestingLevel + 1,
-          node: node.children[i],
+          node: children[i],
         });
       }
     }