We don't necessarily have a child, so debugDescribeChildren() needs to check for null.

sky/sdk/lib/framework/rendering/object.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../node.dart';
 import '../scheduler.dart' as scheduler;
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 class ParentData {
@@ skipping 247 matching lines @@
       adoptChild(_child);
   }
   void attachChildren() {
     if (_child != null)
       _child.attach();
   }
   void detachChildren() {
     if (_child != null)
       _child.detach();
   }
-  String debugDescribeChildren(String prefix) => '${prefix}child: ${child.toString(prefix)}';
+  String debugDescribeChildren(String prefix) {
+    if (child != null)
+      return '${prefix}child: ${child.toString(prefix)}';
+    return '';
+  }
 }
 
 
 // GENERIC MIXIN FOR RENDER NODES WITH A LIST OF CHILDREN
 
 abstract class ContainerParentDataMixin<ChildType extends RenderObject> {
   ChildType previousSibling;
   ChildType nextSibling;
   void detachSiblings() {
     if (previousSibling != null) {
@@ skipping 144 matching lines @@
     while (child != null) {
       if (result != '')
         result += '\n';
       result += '${prefix}child ${count}: ${child.toString(prefix)}';
       count += 1;
       child = child.parentData.nextSibling;
     }
     return result;
   }
 }