fixed regression in NodeTraverser

CHANGELOG

@@ -1,6 +1,6 @@
 * 1.40.1 (2019-XX-XX)
 
-* n/a
+* fixed regression in NodeTraverser
 
 * 1.40.0 (2019-04-28)
 

src/NodeTraverser.php

@@ -69,6 +69,10 @@ class NodeTraverser
         $node = $visitor->enterNode($node, $this->env);
 
         foreach ($node as $k => $n) {
+            if (null === $n) {
+                continue;
+            }
+
             if (false !== ($m = $this->traverseForVisitor($visitor, $n)) && null !== $m) {
                 if ($m !== $n) {
                     $node->setNode($k, $m);

test/Twig/Tests/NodeTraverserTest.php

@@ -0,0 +1,47 @@
+<?php
+
+/*
+ * This file is part of Twig.
+ *
+ * (c) Fabien Potencier
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Twig\NodeTraverser;
+use Twig\Node\Node;
+use Twig\Environment;
+use Twig\NodeVisitor\NodeVisitorInterface;
+
+class Twig_Tests_NodeTraverserTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @group legacy
+     */
+    public function testNodeIsNullWhenTraversing()
+    {
+        $env = new Environment($this->getMockBuilder('\Twig\Loader\LoaderInterface')->getMock());
+        $traverser = new NodeTraverser($env, [new IdentityVisitor()]);
+        $n = new Node([new Node([]), null, new Node([])]);
+        $this->assertCount(3, $traverser->traverse($n));
+    }
+}
+
+class IdentityVisitor implements NodeVisitorInterface
+{
+    public function enterNode(\Twig_NodeInterface $node, Environment $env)
+    {
+        return $node;
+    }
+
+    public function leaveNode(\Twig_NodeInterface $node, Environment $env)
+    {
+        return $node;
+    }
+
+    public function getPriority()
+    {
+        return 0;
+    }
+}