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

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

Index: sky/sdk/lib/framework/rendering/object.dart
diff --git a/sky/sdk/lib/framework/rendering/object.dart b/sky/sdk/lib/framework/rendering/object.dart
index 46b414f3c14e8d032a92e140ac83ae18b559b408..1743f2fdad78e4509a2938dd88e8a7b854b8a377 100644
--- a/sky/sdk/lib/framework/rendering/object.dart
+++ b/sky/sdk/lib/framework/rendering/object.dart
@@ -265,7 +265,11 @@ abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> {
     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 '';
+  }
 }