syncFields() should not sync something you set up in initState(). They are separate concepts.

sky/sdk/lib/widgets/popup_menu.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:async';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/painting/box_painter.dart';
@@ skipping 91 matching lines @@
       backgroundColor: Grey[50],
       borderRadius: 2.0,
       boxShadow: shadows[level]));
     watch(controller.performance);
   }
 
   void syncFields(PopupMenu source) {
     controller = source.controller;
     items = source.items;
     level = source.level;
-    _painter = source._painter;
     super.syncFields(source);
   }
 
   BoxPainter _painter;
 
   double _opacityFor(int i) {
     assert(controller.position.value != null);
     if (controller.position.value == null || controller.position.value == 1.0)
       return 1.0;
     double unit = 1.0 / items.length;
@@ skipping 36 matching lines @@
                 )
               )
             )
           )
         )
       )
     );
   }
 
 }