Don't crash when removing a radio button

sky/sdk/lib/framework/rendering/box.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 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:typed_data';
 import 'object.dart';
 import 'package:vector_math/vector_math.dart';
 import 'package:sky/framework/net/image_cache.dart' as image_cache;
@@ skipping 662 matching lines @@
   RenderCustomPaint({
     CustomPaintCallback callback,
     RenderBox child
   }) : super(child) {
     assert(callback != null);
     _callback = callback;
   }
 
   CustomPaintCallback _callback;
   void set callback (CustomPaintCallback value) {
-    assert(value != null);
+    assert(value != null || !attached);
     if (_callback == value)
       return;
     _callback = value;
     markNeedsPaint();
   }
 
+  void attach() {
+    assert(_callback != null);
+    super.attach();
+  }
+
   void paint(RenderObjectDisplayList canvas) {
+    assert(_callback != null);
     _callback(canvas);
     super.paint(canvas);
   }
 }
 
 // RENDER VIEW LAYOUT MANAGER
 
 class ViewConstraints {
 
   const ViewConstraints({
@@ skipping 92 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }