I was following the Flutter Challenge and got this error when try to hot reload.

Performing hot reload...
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building RadialProgressBar(state: _RadialProgressBarState#c68e1):
flutter: Failed assertion: boolean expression must not be null
flutter:
flutter: Either the assertion indicates an error in the framework itself, or we should provide substantially
flutter: more information in this error message to help you determine and fix the underlying cause.
flutter: In either case, please report this assertion by filing a bug on GitHub:
flutter: https://github.com/flutter/flutter/issues/new
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0 _RenderCustomClip.clipper= (package:flutter/src/rendering/proxy_box.dart)
flutter: #1 ClipOval.updateRenderObject (package:flutter/src/widgets/basic.dart:581:18)
flutter: #2 RenderObjectElement.update (package:flutter/src/widgets/framework.dart:4280:12)
flutter: #3 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4651:11)
flutter: #4 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #5 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #6 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #7 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #8 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #9 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #10 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #11 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #12 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #13 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #14 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #15 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #16 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #17 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #18 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #19 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #20 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #21 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #22 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #23 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #24 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #25 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #26 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #27 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #28 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #29 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #30 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #31 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #32 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #33 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #34 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #35 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #36 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #37 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #38 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #39 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #40 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #41 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #42 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #43 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #44 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #45 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #46 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #47 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #48 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #49 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #50 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #51 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4371:32)
flutter: #52 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4761:17)
flutter: #53 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #54 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #55 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #56 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #57 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #58 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #59 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #60 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #61 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #62 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4371:32)
flutter: #63 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4761:17)
flutter: #64 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #65 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #66 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #67 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #68 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #69 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #70 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #71 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #72 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #73 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #74 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #75 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #76 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #77 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #78 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #79 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #80 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #81 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #82 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #83 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #84 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #85 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #86 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #87 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #88 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #89 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #90 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #91 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #92 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #93 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #94 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #95 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #96 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #97 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #98 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #99 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #100 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #101 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #102 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #103 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #104 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #105 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #106 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #107 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #108 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #109 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #110 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #111 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #112 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #113 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #114 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #115 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #116 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #117 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #118 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #119 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4371:32)
flutter: #120 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4761:17)
flutter: #121 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #122 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #123 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #124 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #125 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #126 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #127 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #128 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #129 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #130 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #131 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #132 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #133 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #134 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #135 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #136 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #137 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #138 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
Reloaded 0 of 393 libraries in 447ms.
flutter: #139 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #140 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #141 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #142 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #143 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #144 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #145 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #146 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #147 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #148 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #149 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #150 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #151 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #152 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #153 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #154 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #155 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #156 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #157 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #158 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #159 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #160 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #161 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #162 StatefulElement.update (package:flutter/src/widgets/framework.dart:3791:5)
flutter: #163 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #164 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #165 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #166 StatelessElement.update (package:flutter/src/widgets/framework.dart:3694:5)
flutter: #167 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #168 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4653:14)
flutter: #169 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #170 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #171 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #172 ProxyElement.update (package:flutter/src/widgets/framework.dart:3901:5)
flutter: #173 Element.updateChild (package:flutter/src/widgets/framework.dart:2691:15)
flutter: #174 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3645:16)
flutter: #175 Element.rebuild (package:flutter/src/widgets/framework.dart:3487:5)
flutter: #176 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2234:33)
flutter: #177 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:626:20)
flutter: #178 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:208:5)
flutter: #179 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)
flutter: #180 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)
flutter: #181 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.scheduleWarmUpFrame. (package:flutter/src/scheduler/binding.dart:751:7)
flutter: #183 _Timer._runTimers (dart:isolate/runtime/libtimer_impl.dart:382:19)
flutter: #184 _Timer._handleMessage (dart:isolate/runtime/libtimer_impl.dart:416:5)
flutter: #185 _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:165:12)
flutter: (elided one frame from package dart:async)
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════