Issue 1:
Flutter environment:

[✓] Flutter (Channel beta, v0.7.3, on Linux, locale en_US.UTF-8)
[!] Android toolchain - develop for Android devices (Android SDK 28.0.1)
✗ Android license status unknown.
[✓] Android Studio (version 3.1)
[✓] IntelliJ IDEA Community Edition (version 2018.2)
[✓] VS Code (version 1.27.1)
[✓] Connected devices (1 available)

Coding context:

calling steps: home widget -> drawer item (routing) -> ONE Widget : successful for the first time.

then back to home widget-> repeat above steps -> bug as follow

Bug information:

I/flutter ( 5796): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 5796): The following assertion was thrown building Builder:
I/flutter ( 5796): 'package:flutter/src/widgets/framework.dart': Failed assertion: line 3736 pos 12: '_state._widget ==
I/flutter ( 5796): null': is not true.
I/flutter ( 5796):
I/flutter ( 5796): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter ( 5796): more information in this error message to help you determine and fix the underlying cause.
I/flutter ( 5796): In either case, please report this assertion by filing a bug on GitHub:
I/flutter ( 5796): https://github.com/flutter/flutter/issues/new
I/flutter ( 5796):
I/flutter ( 5796): When the exception was thrown, this was the stack:
I/flutter ( 5796): #2 new StatefulElement (package:flutter/src/widgets/framework.dart:3736:12)
I/flutter ( 5796): #3 StatefulWidget.createElement (package:flutter/src/widgets/framework.dart:789:42)
I/flutter ( 5796): #4 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2917:40)
I/flutter ( 5796): #5 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #6 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #7 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #8 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #9 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #10 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #11 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #12 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #13 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #14 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #15 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #16 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #17 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #18 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #19 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #20 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #21 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #22 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #23 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #24 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #25 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #26 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #27 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #28 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #29 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #30 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
I/flutter ( 5796): #31 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #32 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #33 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #34 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #35 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #36 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #37 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #38 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #39 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #40 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #41 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #42 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #43 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
I/flutter ( 5796): #44 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #45 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #46 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #47 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #48 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #49 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #50 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #51 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #52 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #53 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #54 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #55 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #56 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #57 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #58 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #59 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #60 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #61 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #62 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
I/flutter ( 5796): #63 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #64 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #65 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #66 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #67 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #68 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #69 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #70 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #71 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #72 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
I/flutter ( 5796): #73 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #74 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #75 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #76 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #77 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #78 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #79 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #80 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #81 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #82 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #83 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #84 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
I/flutter ( 5796): #85 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #86 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #87 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #88 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #89 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #90 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
I/flutter ( 5796): #91 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
I/flutter ( 5796): #92 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
I/flutter ( 5796): #93 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
I/flutter ( 5796): #94 Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
I/flutter ( 5796): #95 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4449:32)
I/flutter ( 5796): #96 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4781:17)
I/flutter ( 5796): #97 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #98 _TheatreElement.update (package:flutter/src/widgets/overlay.dart:507:16)
I/flutter ( 5796): #99 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #100 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #101 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #102 StatefulElement.update (package:flutter/src/widgets/framework.dart:3811:5)
I/flutter ( 5796): #103 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #104 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #105 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #106 ProxyElement.update (package:flutter/src/widgets/framework.dart:3921:5)
I/flutter ( 5796): #107 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #108 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4673:14)
I/flutter ( 5796): #109 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #110 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #111 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #112 StatefulElement.update (package:flutter/src/widgets/framework.dart:3811:5)
I/flutter ( 5796): #113 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #114 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4673:14)
I/flutter ( 5796): #115 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #116 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4673:14)
I/flutter ( 5796): #117 Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
I/flutter ( 5796): #118 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
I/flutter ( 5796): #119 Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
I/flutter ( 5796): #120 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2255:33)
I/flutter ( 5796): #121 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:653:20)
I/flutter ( 5796): #122 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:208:5)
I/flutter ( 5796): #123 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)
I/flutter ( 5796): #124 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)
I/flutter ( 5796): #125 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:842:5)
I/flutter ( 5796): #126 _invoke (dart:ui/hooks.dart:128:13)
I/flutter ( 5796): #127 _drawFrame (dart:ui/hooks.dart:117:3)
I/flutter ( 5796): (elided 2 frames from class _AssertionError)
I/flutter ( 5796): ═════════════════════════════════════════════════════════════════

Issue 2:
For example, I'm trying to find documentation on NetworkImage, to load images from my server.

Google (and search on the flutter.io) both point me to https://docs.flutter.io/flutter/material/Image/Image.network.html which returns 404.