老师，我按照你的列子代码跑起来会报错

The relevant error-causing widget was:
  FutureStudy FutureStudy:file:///Users/gushaohua/study/flutter_net_storage/lib/main.dart:35:19

When the exception was thrown, this was the stack:
#0      _FutureStudyState.build (package:flutter_net_storage/future_study.dart:27:21)
#1      StatefulElement.build (package:flutter/src/widgets/framework.dart:5729:27)
#2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5617:15)
#3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5780:11)
#4      Element.rebuild (package:flutter/src/widgets/framework.dart:5333:7)
#5      BuildScope._tryRebuild (package:flutter/src/widgets/framework.dart:2693:15)
#6      BuildScope._flushDirtyElements (package:flutter/src/widgets/framework.dart:2752:11)
#7      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:3048:18)
#8      WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:1162:21)
#9      RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:468:5)
#10     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1397:15)
#11     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1318:9)
#12     SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:1040:9)
#13     PlatformDispatcher.scheduleWarmUpFrame.<anonymous closure> (dart:ui/platform_dispatcher.dart:837:16)
#17     _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
(elided 3 frames from class _Timer and dart:async-patch)